now , the present invention will be described by referring to the accompanying drawings that illustrate the preferred embodiments of carrying out the invention . fig1 a schematic block diagram of a communication system realized by means of an embodiment of the present invention , illustrating the configuration thereof . an own office has an own office side router 101 , a call control device 102 and first through n - th terminals including a first terminal 103 - 1 , a second terminal 103 - 2 , . . . , an n - th terminal 103 - n . the own office side router 101 , the call control device 102 and the first terminal 103 - 1 , the second terminal 103 - 2 , . . . the n - th terminal 103 - n are connected to a local ip network 104 . the call control device 102 is connected to the first terminal 103 - 1 , the second terminal 103 - 2 , . . . , the n - th terminal 103 - n by way of respective control lines 105 - 1 , 105 - 2 , . . . , 105 - n . the partner office has a partner office side server 112 and a partner office side terminal 113 . the own office side router 101 and the partner office side server 112 are connected to each other by way of a global ip network ( the internet ) 111 . therefore , the partner office side terminal 113 is adapted to communicate with any of the first terminal 103 - 1 , the second terminal 103 - 2 , . . . , the n - th terminal 103 - n by way of the partner office side server 112 , the global ip network 111 and the own office side router 101 . as shown in fig2 , the own office side router 101 is provided with a masquerade table 101 - 1 , an ip masquerade section 101 - 2 and a negotiation section 101 - 3 . the own office side router 101 additionally has functional sections ( not shown ) that an ordinary router is normally provided with . as shown in fig3 , the masquerade table 101 - 1 stores one or more records each describing a relationship between a combination of a global ip address and a global port number and a combination of a local ip address and a local port number . in the example of fig3 , the first record describes that the combination of global ip address 10 . 10 . 20 . 1 and global port number 50001 corresponds to the combination of local ip address 192 . 168 . 0 . 11 and local port number 60001 . in this embodiment , a record may be added to , modified in or erased from the masquerade table 101 - 1 appropriately whenever necessary . the global ip address may be omitted from the masquerade table 101 - 1 . an ip masquerade can be realized even if the global ip address is omitted from the masquerade table 101 - 1 , because it is possible to know which global port number corresponds to which combination of a local ip address and a local port number . the ip masquerade section 101 - 2 conducts an ip masquerade by referring to the masquerade table 101 - 1 . at the time of signaling , the negotiation section 101 - 3 negotiates with the partner office side server 112 . as a result of the negotiation , the partner office side server 112 determines the global port number to be utilized typically by the udp / ip for rtp packet transmission or some other transmission and notifies the determined global port number to record adding section 102 - 5 . the determined global port number is not changed as long as the communication continues . as shown in fig2 , the call control device 102 has an extension number detecting section 102 - 1 , an extension number versus local ip address correspondence table 102 - 2 , a local ip address retrieving section 102 - 3 , a local port number storage section 102 - 4 , a record adding section 102 - 5 , a terminal interface 102 - 6 , a local ip address assigning section 102 - 7 , an origin - of - transfer - related information detecting section 102 - 8 , a record retrieving section 102 - 9 , a destination terminal designation receiving section 102 - 10 , a transfer hold receiving section 102 - 11 , a release terminal detecting section 102 - 12 , a post - transfer local ip address detecting section 102 - 13 , a recording rewriting section 102 - 14 , a record erasing section 102 - 15 , a presence information acquiring section 102 - 16 and a presence information storage section 102 - 17 . when the partner office side terminal 113 designates the direct telephone number of a particular one of the first through n - th terminals 103 - 1 through 103 - n , the extension number detecting section 102 - 1 detects the extension number of the terminal on the basis of the direct telephone number . then , the extension number detection section 102 - 1 requests the terminal interface 102 - 6 to ring a call - in bell of the terminal , using the extension number . when , on the other hand , the partner office side terminal 113 calls the representative telephone number , the extension number detecting section 102 - 1 requests the terminal interface 102 - 6 to ring a call - in bells of all the terminals . then , the extension number detecting section 102 - 1 is notified of the extension number of the terminal that is hooked off first by the terminal interface 102 - 6 . as shown in fig4 , the extension number versus local ip address correspondence table 102 - 2 stores one or more records each describing a relationship between an extension number assigned to a terminal and a local ip address assigned to the terminal . the local ip address retrieving section 102 - 3 receives an extension number from the extension number detecting section 102 - 1 and retrieves the local ip address corresponding to the received extension number from the extension number versus local ip address correspondence table 102 - 2 . the retrieved local ip address then becomes the local ip address of the terminal that is to start a communication . the local port number storage section 102 - 4 holds the local port number that corresponds to the type of the coming communication . at the time of signaling , the record adding section 102 - 5 receives the global ip address and the global port number to be used for the coming communication from the negotiation section 101 - 3 . note , however , that it is not necessary for the record adding section 102 - 5 to receive the global ip address from the negotiation section 101 - 3 when the global ip address of the own office side router 101 is known or when no field is provided for global ip addresses in the masquerade table 101 - 1 . additionally , at the time of signaling , the record adding section 102 - 5 receives the local ip address to be used for the coming communication . ( the local ip address of the terminal to be used for the coming communication ) from the local ip address retrieving section 102 - 3 . furthermore , at the time of signaling , the record adding section 102 - 5 receives the local port number to be used for the coming communication from the local port number storage section 102 - 4 . then , the record adding section 102 - 5 adds a record describing the global ip address , the global port number , the local ip address and the local port number it has received to the masquerade table 101 - 1 . note , however , that the added record describes only the global port number , the local ip address and the local port number when no field is provided for global ip addresses in the masquerade table 101 - 1 . the record adding section 102 - 5 may determine the local port number by negotiating with the terminal with which it is going to communicate . the record adding section 102 - 5 may receive a designated local port number although the record adding section 102 - 5 has the initiative of the negotiation in hand . the terminal interface 102 - 6 communicates with the first terminal 103 - 1 , the second terminal 103 - 2 , . . . , the n - th terminal 103 - n for the purpose of controlling the call . the local ip address assigning section 102 - 7 receives the local ip address of the terminal that is to be newly added as extension from this terminal , adds a new extension number that is generated internally and adds a record describing the new local ip address and the new extension number to the extension number versus local ip address correspondence table 102 - 2 . the origin - of - transfer - related information detecting section 102 - 8 detects the extension number or the local ip address of the terminal requesting transfer of communication . when the origin - of - transfer - related information detecting section 102 - 8 detects the extension number of the terminal requesting transfer of communication , the origin - of - transfer - related information detecting section 102 - 8 retrieves the local ip address corresponding to the extension number from the extension number versus local ip address correspondence table 102 - 2 . additionally , the origin - of - transfer - related information detecting section 102 - 8 detects the type of the communication of which transfer is requested or the local port number that is used for the communication of which transfer is requested . if the origin - of - transfer - related information detecting section 102 - 8 detects the type of the communication of which transfer is requested , it retrieves the local port number corresponding to the type of the communication from the local port number storage section 102 - 4 . if , for example , the type of communication is limited to ip telephony and the local port number is fixed , it does not have to retrieve the local port number but have only to retain the local port number . the origin - of - transfer - related information detecting section 102 - 8 may alternatively inquire the local port number being used for communication to the origin terminal of the transfer . the record retrieving section 102 - 9 receives the local ip address and the local port number of the origin of transfer from the origin - of - transfer - related information detecting section 102 - 8 and retrieves the record describing the received local ip address and local port number from the masquerade table 101 - 1 . then , the record retrieving section 102 - 9 notifies the record rewriting section 102 - 14 of information for identifying the retrieved record ( including the record number ). two methods can be used for transfer of communication as described below . the first one is that the origin of transfer designates the destination of transfer . more specifically , the origin terminal of transfer holds the communication and notifies to the designated destination of transfer by way of extension that the origin terminal of transfer transfers the communication to the designated destination of transfer . then , the origin terminal of transfer selects the designated destination of transfer after disconnecting the extension . the communication resumes as the designated destination of transfer release the communication that has been held . the second method is that the origin of transfer does not designate the destination of transfer . more specifically , the origin terminal of transfer holds the communication and the terminal that firstly releases the communication that has been held becomes the destination terminal of transfer . the destination terminal designation receiving section 102 - 10 is adapted to operate for the first transfer method and receive the extension number of the destination terminal of transfer from the origin terminal of transfer . the transfer hold receiving section 102 - 11 and the release terminal detecting section 102 - 12 are adapted to operate for the second transfer method . the transfer hold receiving section 102 - 11 receives a hold of communication by the origin terminal of transfer and notifies the release terminal detecting section 102 - 12 of that the communication is in a held state . the release terminal detecting section 102 - 12 detects the terminal that is firstly hooked off first as a release terminal . the post - transfer local ip address detecting section 102 - 13 receives the extension number of the destination terminal of transfer from the destination terminal designation receiving section 102 - 10 when the first transfer method is used whereas the post - transfer local ip address detecting section 102 - 13 receives the extension number of the destination terminal of transfer from the release terminal detecting section 102 - 12 when the second transfer method is used . then , the post - transfer local ip address detecting section 102 - 13 retrieves the local ip address that corresponds to the received extension number from the extension number versus local ip address correspondence table 102 - 2 . the record rewriting section 102 - 14 receives information for specifying the record to be rewritten out of the records of the masquerade table 101 - 1 and the local ip address to be written to the record to be rewritten ( the local ip address of the destination terminal of transfer ) respectively from the record retrieving section 102 - 9 and the post - transfer local ip address detecting section 102 - 13 and rewrites the record to be rewritten among the records of the masquerade table 101 - 1 so as to replace the local ip address in the record by the local ip address of the destination terminal of transfer . as a result of this rewriting , the transfer of communication is realized . in principle , it is not necessary to rewrite the local port number if the local port number is determined for the type of the communication that is taking place . however , if it is necessary to change the local port number , the record rewriting section also rewrites the local port number by the new local port number on the record to be rewritten among the records of the masquerade table 101 - 1 . it is necessary to change the local port number , for example , when the local port number is designated by the destination terminal of transfer in the negotiation with the destination terminal of transfer or when the default local port number that is to be used by the destination terminal of transfer is occupied at present . the record erasing section 102 - 15 detects the extension number or the local ip address of the terminal that has terminated a communication . if the record erasing section 102 - 15 detects the extension number of the terminal that has terminated a communication , the record erasing section 102 - 15 retrieves the local ip address that corresponds to the extension number from the extension number versus local ip address correspondence table 102 - 2 . the record erasing section 102 - 15 also detects the type of the communication that has been terminated or the local port number used for the communication that has been terminated . if the record erasing section 102 - 15 detects the type of the communication that has been terminated , the record erasing section 102 - 15 retrieves the local port number that corresponds to the type of the communication from the local port number storage section 102 - 4 . if , for example , the type of communication is limited to ip telephony and the local port number is fixed , the record erasing section 102 - 15 does not have to detect nor retrieve the local port number but only has to retain the local port number . the record erasing section 102 - 15 then erases the record that describes the detected or retrieved local ip address and the detected or retrieved local port number from the masquerade table 101 - 1 . by doing so , the unnecessary port is closed to improve the security and prevent the unnecessary record from remaining in the masquerade table 101 - 1 . the presence information storage section 102 - 17 stores information on the current presence or absence of the user at each terminal . it also stores information on the priority of the available terminals for each user . information for identifying a terminal may be the extension number or the local ip address of the terminal . if a terminal is identified by the extension number thereof , the information stored in the presence information storage section 102 - 17 will be such as illustrated in fig5 . if , on the other hand , a terminal is identified by the local ip address thereof , the information stored in the presence information storage section 102 - 17 will be such as illustrated in fig6 . the presence information acquiring section 102 - 16 acquires the information to be stored in the presence information storage section 102 - 17 from each terminal by way of the terminal interface 102 - 6 . alternatively , the presence information acquiring section 102 - 16 may acquire the information to be stored in the presence information storage section 102 - 17 from each terminal by way of local ip network 104 . if a terminal is identified by the extension number thereof by way of the presence information storage section 102 - 17 , the extension number detecting section 102 - 1 may operate in a manner as described below , using presence information , in addition to the above described operation . when a call specifying a particular terminal is received and it is made clear from the information stored in the presence information storage section 102 - 17 that the user is currently absent at the terminal , the terminal having the second degree of priority is selected as terminal for receiving the call out of the terminals available to the user and the extension number of the terminal is delivered to the local ip address retrieving section 102 - 3 . this operation is repeated until one of terminals concerned is hooked off or the call gets to the terminal having the lowest degree of priority . when a call specifying a particular terminal is received and it is made clear from the information stored in the presence information storage section 102 - 17 that the user is supposed to be currently present at the terminal but the terminal is not hooked off within a predetermined period of time ( e . g ., 30 seconds ), the terminal having the second degree of priority is selected as the terminal for receiving the call out of the terminals available to the user and the extension number of the terminal is delivered to the local ip address retrieving section 102 - 3 . this operation is repeated until one of terminals concerned is hooked off or the call gets to the terminal having the lowest degree of priority . if a terminal is identified by the local ip address thereof in the presence information storage section 102 - 17 , the local ip address retrieving section 102 - 3 may operate in a manner as described below , using presence information , in addition to the above described operation . when a call specifying a particular terminal is received and , as a result of receiving the extension number from the extension number detecting section 102 - 1 and retrieving the local ip address corresponding to the extension number from the extension number versus local ip address correspondence table 102 - 2 , it is made clear from the information stored in the presence information storage section 102 - 17 that the user is currently absent at the terminal , the terminal having the second degree of priority is selected as terminal for receiving the call out of the terminals available to the user and the local ip address of the terminal is delivered to the record adding section 102 - 5 . this operation is repeated until one of terminals concerned is hooked off or the call gets to the terminal having the lowest degree of priority . when a call specifying a particular terminal is received and , as a result of receiving the extension number from the extension number detecting section 102 - 1 and retrieving the local ip address corresponding to the extension number from the extension number versus local ip address correspondence table 102 - 2 , it is made clear from the information stored in the presence information storage section 102 - 17 that the user is supposed to be currently present at the terminal but the terminal is not hooked off within a predetermined period of time ( e . g ., 30 seconds ), the terminal having the second degree of priority is selected as the terminal for receiving the call out of the terminals available to the user and the local ip address of the terminal is delivered to the record adding section 102 - 5 . this operation is repeated until one of terminals concerned is hooked off or the call gets to the terminal having the lowest degree of priority . now , the operation of the own office side router 101 and that of the call control device 102 at the time of starting a communication will be described below by referring to fig7 . firstly , the negotiation section 101 - 3 negotiates with the partner office side server 112 to determine the global port number to be used for the communication ( step s 201 ). then , the extension number detecting section 102 - 1 detects the extension number of the terminal to be used for the communication ( step s 202 ). then , the local ip address retrieving section 102 - 3 retrieves the local ip address that corresponds to the extension number detected in step s 202 from the extension number versus local ip address correspondence table 102 - 2 ( step s 203 ). the local ip address is that of the terminal to be used for the communication . thereafter , the record adding section 102 - 5 determines the local port number to be used for the communication ( step s 204 ). then , the record adding section 102 - 5 adds a record describing the corresponding relationship among the global port number determined in step s 201 , the local ip address retrieved in step s 203 and the local port number determined in step s 204 to the masquerade table 101 - 1 ( step s 205 ). now , the operation of the own office side router 101 and that of the call control device 102 at the time of transferring the communication will be described below by referring to fig8 . firstly , the origin - of - transfer - related information detecting section 102 - 8 detects the extension number of the origin terminal of transfer ( step s 211 ). then , the origin - of - transfer - related information detecting section 102 - 8 retrieves the local ip address that corresponds to the extension number detected in step s 211 from the extension number versus local ip address correspondence table 102 - 2 ( step s 212 ). alternatively , the origin - of - transfer - related information detecting section 102 - 8 may directly detect the local ip address of the origin terminal of transfer in place of the detecting operation of steps s 211 and s 212 ( step s 213 ). then , the origin - of - transfer - related information detecting section 102 - 8 detects the local port number that is being used for the communication ( step s 214 ). thereafter , the record retrieving section 102 - 9 retrieves the information for identifying the record describing the local ip address retrieved in step s 212 or in step s 213 and the local port number detected in step s 214 from the masquerade table 101 - 1 ( step s 215 ). the record is to be rewritten . subsequently , the destination terminal designation receiving section 102 - 10 or the release terminal detecting section 102 - 12 detects the extension number of the destination terminal of transfer ( step s 216 ). then , the post - transfer local ip address detecting section 102 - 13 retrieves the local ip address that corresponds to the extension number of the destination of transfer detected in step s 216 from the extension number versus local ip address correspondence table 102 - 2 ( step s 217 ). alternatively , the destination terminal designation receiving section 102 - 10 or the release terminal detecting section 102 - 12 may directly detect the local ip address of the destination terminal of transfer in place of the detecting operation of steps s 216 and s 217 ( step s 218 ). then , the record rewriting section 102 - 14 determines the post - transfer local port number ( step s 219 ). then , the record rewriting section 102 - 14 rewrites the record identified by the identifying information as retrieved in step s 215 so as to replace the local ip address and the local port number in the record respectively by the local ip address retrieved in step s 217 or detected in step s 218 and the local port number determined in step s 219 ( step s 220 ). next , the processing operation of the own office side router 101 and that of the call control device 102 at the time of automatic transfer according to the presence information and by means of the above described first method will be described by referring to fig9 . firstly , the negotiation section 101 - 3 negotiates with the partner office side server 112 to determine the global port number to be used for the communication ( step s 201 ). then , the extension number detecting section 102 - 1 determines whether an extension number is designated ( step s 242 ). if there is not any designated extension number ( no in step s 242 ), the extension number detecting section 102 - 1 detects the extension number of the terminal that is hooked off first ( step s 243 ) and proceeds to step s 203 ( fig7 ). if , on the other hand , there is a designated extension number ( yes in step s 242 ), the extension number detecting section 102 - 1 detects the designated extension number ( step s 244 ) and determines whether the user is present at the terminal having the extension number detected in step s 244 by referring to the information stored in the presence information storage section 102 - 17 ( step s 245 ). if the user is absent at the terminal having the extension number ( yes in step s 245 ), the processing operation proceeds to step s 248 . if , on the other hand , the user is present at the terminal having the extension number ( no in step s 245 ), a call - in bell of the terminal having the extension number detected in step s 244 is rung up ( step s 246 ). if the terminal is hooked off within a predetermined period of time ( no in step s 247 ), the processing operation proceeds to step s 203 ( fig7 ). on the other hand , if the terminal is not hooked off within the predetermined period of time ( yes in step s 247 ), the processing operation proceeds to step s 248 . in step s 248 , it is determined whether the current extension number has the lowest degree of priority by referring to the information stored in the presence information storage section 102 - 17 and , if the degree of priority is the lowest one ( yes in step s 248 ), the processing operation is terminated . simultaneous call in may be used in place of terminating the processing operation . if the degree of priority is not the lowest one ( no in step s 248 ), the current extension number is switched to the extension number of the terminal having the next degree of priority by referring to the information stored in the presence information storage section 102 - 17 ( step s 249 ) and the processing operation returns to step s 245 . now , the processing operation of the own office side router 101 and that of the call control device 102 at the time of automatic transfer according to the presence information and by means of the above described second method will be described by referring to fig1 . firstly , the negotiation section 101 - 3 negotiates with the partner office side server 112 to determine the global port number to be used for the communication ( step s 201 ). then , the extension number detecting section 102 - 1 determines whether an extension number is designated ( step s 242 ). if there is not any designated extension number ( no in step s 242 ), the extension number detecting section 102 - 1 detects the extension number of the terminal that is hooked off first ( step s 243 ) and proceeds to step s 203 ( fig7 ). on the other hand , if there is a designated extension number ( yes in step s 242 ), the extension number detecting section 102 - 1 detects the designated extension number ( step s 244 ). then , the local ip address retrieving section 102 - 3 retrieves the local ip address corresponding to the extension number that is detected in step s 244 from the extension number versus local ip address correspondence table 102 - 2 ( step s 251 ). then , it is determined whether the user is present at the terminal having the local ip address retrieved in step s 251 by referring to the information stored in the presence information storage section 102 - 17 ( step s 252 ). if the user is absent at the terminal having the extension number ( yes in step s 252 ), the processing operation proceeds to step s 255 . if , on the other hand , the user is present at the terminal having the extension number ( no in step s 252 ), a call - in bell of the terminal having the extension number detected in step s 244 is rung up ( step s 253 ). if the terminal is hooked off within a predetermined period of time ( no in step s 254 ), the processing operation proceeds to step s 204 ( fig7 ). on the other hand , if the terminal is not hooked off within the predetermined period of time ( yes in step s 254 ), the processing operation proceeds to step s 255 . in step s 255 , it is determined whether the current local ip address has the lowest degree of priority by referring to the information stored in the presence information storage section 102 - 17 and , if the degree of priority is the lowest one ( yes in step s 255 ), the processing operation is terminated . simultaneous call in may be used in place of terminating the processing operation . if the degree of priority is not the lowest one ( no in step s 255 ), the current local ip address is switched to the local ip address of the terminal having the next degree of priority by referring to the information stored in the presence information storage section 102 - 17 ( step s 256 ) and the processing operation returns to step s 252 . the call control device 102 acquires information for identifying the record to be rewritten in the masquerade table 101 and then the record is designated by the identifying information so as to rewrite the record in the first embodiment . however , if the own office side router 101 and the call control device 102 communicate with each other according to the upnp ( universal plug and play ) standard , it is not possible to acquire information necessary for identifying the record describing a combination of a particular local ip address and a particular local port number out of the records in the masquerade table 101 - 1 . additionally , if the own office side router 101 and the call control device 102 communicate with each other according to the upnp standard , it is not possible for the call control device 102 to rewrite any record in the masquerade table 101 - 1 . on the other hand , if the own office side router 101 and the call control device 102 communicate with each other according to the upnp standards , it is possible for the call control device 102 to erase a record designated by a global port number from the masquerade table 101 - 1 and add a record describing a desired corresponding relationship of a combination of a global ip address and a pair of a local ip address and a local port number to the masquerade table 101 - 1 . therefore , the second embodiment is so adapted that no record in the masquerade table 101 - 1 is rewritten but any desired record can be erased and a new record can be added to replace the erased record in order to achieve an effect equivalent to that of rewriting a record . once a record is erased , no record corresponding to the erased record exists until a new record is added as replacement . however , no problem arises because no communication takes place between the erasure of a record and the addition of a replacement record . as shown in fig1 , the own office side router 101 of the second embodiment has a configuration similar to that of the own office side router 101 of the first embodiment . as shown in fig1 , the call control device 102 of the second embodiment is partly identical with the call control device 102 of the first embodiment but partly different from the latter . more specifically , the call control device 102 of the second embodiment has a replica 102 - 50 of the masquerade table , an extension number detecting section 102 - 51 , an extension number versus local ip address correspondence table 102 - 52 , a local ip address retrieving section 102 - 53 , a local port number storage section 102 - 54 , a record adding section 102 - 55 , a terminal interface 102 - 56 , a local ip address assigning section 102 - 57 , an origin - of - transfer - related information detecting section 102 - 58 , a record retrieving section 102 - 59 , a destination terminal designation receiving section 102 - 60 , a transfer hold receiving section 102 - 61 , a release terminal detecting section 102 - 62 , a post - transfer local ip address detecting section 102 - 63 , a record erasing section 102 - 65 , a presence information acquiring section 102 - 66 and a presence information storage section 102 - 67 . the replica 102 - 50 of the masquerade table has a configuration same as that of the masquerade table 101 - 1 and stores the records same as those of the latter . the replica 102 - 50 of the masquerade table does not have a field for global ip addresses like the masquerade table 101 - 1 . in the following description , it is assumed that there is no field for global ip addresses . the extension number detecting section 102 - 51 is similar to the extension number detecting section 102 - 1 of the first embodiment . the extension number versus local ip address correspondence table 102 - 52 is similar to the extension number versus local ip address correspondence table 102 - 2 of the first embodiment . the local ip address retrieving section 102 - 53 is similar to the local ip address retrieving section 102 - 3 of the first embodiment the local port number storage section 102 - 54 is similar to the local port number storage section 102 - 4 of embodiment 1 . at the time of signaling , the record adding section 102 - 55 receives the global ip address and the global port number to be used for the coming communication from the negotiation section 101 - 3 . note , however , that it is not necessary for the record adding section 102 - 55 to receive the global ip address from the negotiation section 101 - 3 if the global ip address of the own office side router 101 is known or if no field is provided for global ip addresses in the masquerade table 101 - 1 and the replica 102 - 50 of the masquerade table . additionally , at the time of signaling , the record adding section 102 - 55 receives the local ip address to be used for the coming communication ( the local ip address of the terminal to be used for the coming communication ) from the local ip address retrieving section 102 - 53 . furthermore , at the time of signaling , the record adding section 102 - 55 receives the local port number to be used for the coming communication from the local port number storage section 102 - 54 . then , the record adding section 102 - 55 adds a record describing the global ip address , the global port number , the local ip address and the local port number it received to the masquerade table 101 - 1 and the replica 102 - 50 of the masquerade table . note , however , that the added record describes only the global port number , the local ip address and the local port number if no field is provided for global ip addresses in the masquerade table 101 - 1 and the replica 102 - 50 of the masquerade table . additionally , after the record erasing section 102 - 65 has erased a record , the record adding section 102 - 55 subsequently operates for transfer of communication in a manner as described below . namely , the record adding section 102 - 55 receives the global port number to be described in the record to be added from the record retrieving section 102 - 59 . additionally , the record adding section 102 - 55 receives the post - transfer local ip address to be described in the record to be added from the post - transfer local ip address detecting section 102 - 63 . furthermore , the record adding section 102 - 55 retrieves the local port number to be described in the record to be added from the local port number storage section 102 - 54 depending on the type of communication . note , however , the local port number to be described in the record to be added may be determined by negotiating with the destination terminal of transfer . then , the record adding section 102 - 55 adds the received global port number , the received local ip address and the retrieved or determined local port number to the masquerade table 101 - 1 and the replica 102 - 50 of the masquerade table . the terminal interface 102 - 56 is similar to the terminal interface 102 - 6 of the first embodiment . the local ip address assigning section 102 - 57 is similar to the local ip address assigning section 102 - 7 of the first embodiment . the origin - of - transfer - related information detecting section 102 - 58 is similar to the origin - of - transfer - related information detecting section 102 - 8 of the first embodiment . the record retrieving section 102 - 59 receives the local ip address and the local port number of the origin of transfer from the origin - of - transfer - related information detecting section 102 - 58 and retrieves the record describing the local ip address and the local port number from the replica 102 - 50 of the masquerade table . then , the record retrieving section 102 - 59 notifies the record erasing section 102 - 65 and the record adding section 102 - 55 of the global port number described in the retrieved record . the destination terminal designation receiving section 102 - 60 is similar to the destination terminal designation receiving section 102 - 10 of the first embodiment . the transfer hold receiving section 102 - 61 is similar to the transfer hold receiving section 102 - 11 of the first embodiment . the release terminal detecting section 102 - 62 is similar to the release terminal detecting section 102 - 12 of the first embodiment . the post - transfer local ip address detecting section 102 - 63 is similar to the post - transfer local ip address detecting section 102 - 13 of the first embodiment . at the time of transfer of communication , the record erasing section 102 - 65 operates in a manner as described below . namely , the record erasing section 102 - 65 acquires the global port number , described in the record that describes the local ip address and the local port number of the origin of transfer , from the record retrieving section 102 - 59 . then , the record erasing section 102 - 65 erases the record that describes the global port number acquired from the record retrieving section 102 - 59 from the masquerade table 101 - 1 and the replica 102 - 50 of the masquerade table . at the time of end of communication , the record erasing section 102 - 65 operates in a manner as described below . namely , the record erasing section 102 - 65 detects the extension number or the local ip address of the terminal that has terminated a communication . if the record erasing section 102 - 65 detects the extension number of a terminal that has terminated a communication , the record erasing section 102 - 65 retrieves the local ip address corresponding to the extension number from the extension number versus local ip address correspondence table 102 - 52 . the record erasing section 102 - 65 also detects the type of the communication that has been terminated or the local port number used for the communication that has been terminated . if the record erasing section 102 - 65 detects the type of the communication that has been terminated , the record erasing section 102 - 65 retrieves the local port number that corresponds to the type of the communication from the local port number storage section 102 - 54 . if , for example , the type of communication is limited to ip telephony and the local port number is fixed , the record erasing section 102 - 65 does not have to detect nor retrieve the local port number but only has to retain the local port number . then , the record erasing section 102 - 65 requests the record retrieving section 102 - 59 to retrieve the global port number described in the record that describes the detected or retrieved local ip address and the detected or retrieved local port number from the replica 102 - 50 of the masquerade table and acquires the global port number retrieved by the record retrieving section 102 - 59 . then , the record erasing section 102 - 65 erases the record that describes the global port number acquired from the record retrieving section 102 - 59 from the masquerade table 101 - 1 and the replica 102 - 50 of the masquerade table . by doing so , the unnecessary port is closed to improve the security and prevent the unnecessary record from remaining in the masquerade table 101 - 1 and the replica 102 - 59 of the masquerade table . the presence information acquiring section 102 - 66 is similar to the presence information acquiring section 102 - 16 of the first embodiment . the presence information storage section 102 - 67 is similar to the presence information storage section 102 - 17 of the first embodiment . next , the operation of the own office side router 101 and that of the call control device 102 at the time of starting a communication will be described below by referring to fig1 . firstly , the negotiation section 101 - 3 negotiates with the partner office side server 112 to determine the global port number to be used for the communication ( step s 301 ). then , the extension number detecting section 102 - 51 detects the extension number of the terminal to be used for the communication ( step s 302 ). then , the local ip address retrieving section 102 - 53 retrieves the local ip address that corresponds to the extension number detected in step s 302 from the extension number versus local ip address correspondence table 102 - 52 ( step s 303 ). the local ip address is that of the terminal to be used for the communication . thereafter , the record adding section 102 - 55 determines the local port number to be used for the communication ( step s 304 ). then , the record adding section 102 - 55 adds a record describing a relationship between the global port number determined in step s 301 and a pair of the local ip address retrieved in step s 303 and the local port number determined in step s 304 to the masquerade table 101 - 1 ( step s 305 ). then , the record adding section 102 - 55 adds a record identical with the record added to the masquerade table 101 - 1 in step s 305 to the replica 102 - 50 of the masquerade table ( step s 306 ). now , the operation of the own office side router 101 and that of the call control device 102 of the second embodiment 2 at the time of transferring the communication will be described below by referring to fig1 . firstly , the origin - of - transfer - related information detecting section 102 - 58 detects the extension number of the origin terminal of transfer ( step s 311 ). then , the origin - of - transfer - related information detecting section 102 - 58 retrieves the local ip address that corresponds to the extension number detected in step s 311 from the extension number versus local ip address correspondence table 102 - 52 ( step s 312 ). alternatively , the origin - of - transfer - related information detecting section 102 - 58 may directly detect the local ip address of the origin terminal of transfer in place of the detecting operation of steps s 311 and s 312 ( step s 313 ). then , the origin - of - transfer - related information detecting section 102 - 58 detects the local port number that is being used for the communication ( step s 314 ). thereafter , the record retrieving section 102 - 59 retrieves the global port number described in the record that describes the local ip address retrieved in step s 312 or detected in step s 313 and the local port number detected in step s 314 from the replica 102 - 50 of the masquerade table ( step s 315 ). then , the record erasing section 102 - 65 erases the record describing the global port number retrieved in step s 315 from the masquerade table 101 - 1 ( step s 316 ) and the record describing the global port number retrieved in step s 315 from the replica 102 - 50 of the masquerade table ( step s 317 ). subsequently , the destination terminal designation receiving section 102 - 60 or the release terminal detecting section 102 - 62 detects the extension number of the destination terminal of transfer ( step s 318 ). then , the post - transfer local ip address detecting section 102 - 63 retrieves the local ip address that corresponds to the extension number of the destination of transfer detected in step s 318 from the extension number versus local ip address correspondence table 102 - 52 ( step s 319 ). alternatively , the destination terminal designation receiving section 102 - 60 or the release terminal detecting section 102 - 62 may directly detect the local ip address of the destination terminal of transfer in place of the detecting operation of steps s 318 and s 319 ( step s 320 ). then , the record adding section 102 - 55 determines the post - transfer local port number ( step s 321 ). then , the record adding section 102 - 55 adds a record describing the global port number retrieved in step s 315 , the local ip address retrieved in step s 319 or the local ip address detected in step s 320 and the local port number determined in step s 321 to the masquerade table 101 - 1 ( step s 322 ) and a record describing the same contents as the record added in step s 322 to the replica 102 - 50 of the masquerade table ( step s 323 ). next , the processing operation of the own office side router 101 and that of the call control device 102 of embodiment 2 at the time of automatic transfer according to the presence information and by means of the above described first method will be described by referring to fig1 . it will be clear by comparing fig9 and fig1 that a processing operation same as that of the first embodiment takes place in the second embodiment . note , however , that the processing operation proceeds to step s 303 ( fig1 ) after step s 243 if the outcome of determination in step s 242 is no and proceeds to step s 303 ( fig1 ) if the outcome of determination in step s 247 is no . now , the processing operation of the own office side router 101 and that of the call control device 102 of the second embodiment 2 at the time of automatic transfer according to the presence information and by means of the above described second method will be described by referring to fig1 . it will be clear by comparing fig1 and fig1 that a processing operation same as that of the first embodiment takes place in the second embodiment 2 . note , however , that the processing operation proceeds to step s 303 ( fig1 ) after step s 243 if the outcome of determination in step s 242 is no and proceeds to step s 304 ( fig1 ) if the outcome of determination in step s 254 is no . to have the person at the partner office hear a melody representing hold , it is only necessary to assign a local ip address and a local port number to a sound source of the melody representing hold and transfer it to the local ip address and the local port number .