[ 0048 ] fig2 shows switch devices and a system using these switches according to an embodiment of the present invention . referring to fig2 pbxs 30 and 32 , which are switch devices , are connected to an isdn 34 , which is a public network . an isdn tie line 36 connects the pbxs 30 and 32 together . terminals such as telephone sets are connected to the pbxs 30 and 32 so that a pbx network which is a private network is formed . for the sake of simplicity , only one telephone set 38 connected to the pbx 30 and assigned an id “ a ” is illustrated , and only one telephone set 40 connected to the pbx 32 and assigned an id “ b ” is illustrated . telephone sets 42 and 44 respectively assigned ids “ x ” and “ y ” are connected to the isdn 34 . [ 0049 ] fig3 is a block diagram of each of the pbxs 30 and 32 shown in fig2 . the pbx includes line control parts 46 , 48 and 50 , a network ( switch ) part 52 , a controller ( cc ) 54 formed of , for example , a processor or a cpu , and a main memory ( mm ) 56 . the line control part 46 includes btk ( isdn bri switch office trunk ) cards , and ptk ( isdn pri switch office trunk ) cards , and controls a line connection with the isdn 34 . the line control part 48 includes dtlc ( digital terminal line circuit ) cards and blc ( bri line circuit ) cards , and controls a line connection with the extension telephone set 38 or the like . the line control part 50 includes itk trunks ( isdn tie line trunks ), and controls a line connection with the isdn tie line 36 . the controller 54 performs a message translation control in addition to the above line connection control . the message translation control is carried out by a message receive part 60 , an information element extracting part 61 , an information element control part 62 , an information element editing part 63 , and a message transmit part 64 . the main memory 56 includes a dil ( direct in line ) translation table 65 storing information necessary for line connections , and an extension dial - in number translation table 66 . the controller 54 refers to the dil translation table 65 when performing the line connection control . the table 66 is used when the controller 54 executes the message translation control . [ 0052 ] fig4 and 5 show formats of call setup messages ( setup ) transferred over the isdn tie line 36 . each of the messages includes two code sets named code set # 0 # and code set # 7 . code set ( which can be described as codeset ) # 0 of the call setup message shown in fig4 includes a calling id information element m 1 for discriminating a calling terminal , and a called id information element m 2 for discriminating a called terminal . code set # 7 , which is an additional code set , includes calling id information element m 3 for discriminating a call terminal connected to the isdn 34 . the above two code sets are defined in the recommendation q . 931 , the disclosure of which is hereby incorporated by reference . code set # 0 of the call setup message shown in fig5 includes the calling id information element m 1 and the called id information element m 2 . code set # 7 serves as an additional service information element , and includes a pbx extension dial - in number m 4 , which may be the dial - in numbers of the extension telephone sets 48 and 40 connected to the pbxs 30 and 32 , respectively . the message receive part 60 receives a message from the line control part 46 or 50 . the received message is sent to the information element extracting part 61 . if the received message is the above - mentioned call setup message , the information element extracting part 61 extracts therefrom the calling id information element m 1 , the called id information element m 2 , the calling id information element m 3 and the dial - in number m 4 . the extracted information elements are supplied to the information element control part 62 . the information element control part 62 searches the dil translation table 65 by using the called id information element m 2 included in the message sent from the line control part 46 or 50 , and translates it into the number of the pbx ( pbx number ) and the extension number . if a call is generated by an extension telephone set via the line control part 48 , the information element control part 62 searches the extension dial - in number translation table 66 , and obtains the dial - in number of the above extension telephone set . the information element editing part 63 writes the pbx number and the extension number obtained by searching the table 66 into the called id information element m 2 of the call setup message . further , the information element editing part 63 writes the extracted calling id information element m 1 into the calling id information element m 3 of the call setup message . furthermore , the information element editing part 63 writes the extracted calling id information element m 3 into the calling id information element m 1 of the call setup message . moreover , the information element editing part 63 writes the extension number obtained by searching the extension dial - in number translation table 66 into the dial - in number m 4 of the call setup message . the information element editing part 63 writes the extracted dial - in number m 4 into the calling id information element m 1 of the call setup message . then , the message transmit part 64 sends the message thus edited to the line control part 46 or 50 . [ 0056 ] fig6 a and 6b are flowcharts of a sequence in which the telephone set 42 generates a call which should be sent to the isdn 34 , the pbx 30 , the isdn tie line 36 and the pbx 32 . in this sequence , there are two cases , in one of which cases the call is addressed to the extension telephone set 40 accommodated by the pbx 32 . in the other case , the call is transferred via the pbx 32 and is addressed to the telephone set 44 connected to the isdn . first , the case where a call is addressed to the extension telephone set 40 will be described with reference to fig7 in which a call is transferred as indicated by the broken line . fig8 will also be referred to in order to explain how information is transferred . step numbers shown in fig7 correspond to those shown in fig8 . further , fig9 which explains the call setup message will be referred to . at step s 10 , the telephone set 42 connected to the isdn 34 sends a call to the pbx 30 . hence , a call setup message sm 1 shown in fig9 is sent to the pbx 30 from the isdn network 34 . the call setup message sm 1 includes “ 044777 xxxx ” and “ 044777 abcd ”, which are respectively the calling id information element and the called id information element of code set # 0 . at step s 12 , the call arrives at the pbx 30 . at step s 14 , the controller 54 of the pbx 30 extracts the calling id information element and the called id information element from code set # 0 of the call setup message sm 1 . at step s 16 , the controller 54 refers to the dil translation table 65 , and translates the extracted called id information element into the called id information via the pbx 32 . in the present example , “ 044777 abcd ” is translated into “ 710 + zzzz ”. at step s 18 , the controller 54 refers to the called id information and thus identifies the remote terminal to which the call should be connected . in the present case , the called to which the call should be connected is the isdn tie line 36 . thus , the process proceeds to step s 20 . at step s 20 , the controller 54 assembles a call setup message for the isdn tie line 36 so that it includes the calling id information element m 1 “ 700 ”, prefix dial number “ 0 ”, and the extracted calling id information element “ 044777 xxxx ”. the above “ 700 ” is the number of the pbx 32 to which the call should be transferred . the above prefix dial number “ 0 ” indicates a connection of a line between the pbx 30 and the isdn 34 . further , the via - pbx 32 called id information element “ 710 + zzzz ” obtained at step s 16 is written into the called id information element of code set # 0 of the call setup message sm 2 . the calling id information element “ 044777 xxxx ” extracted at step s 14 is written into the calling id information element m 3 of code set # 7 . at step s 22 , the call setup message sm 2 thus assembled is sent to the pbx 32 . at step s 24 , the controller 54 of the pbx 32 extracts the calling id information element “ 044777 xxxx ” of the telephone set 42 from the calling id information element of code set # 7 of the call setup message transferred over the isdn tie line 36 , and extracts the called id information element of code set # 0 . at step s 26 , the controller 54 identifies the connection called from the extracted called id information element . in the present case , the connection called is the extension telephone set 40 of the pbx 32 , and thus the process proceeds to step s 28 . at step s 28 , the calling id information of the telephone set 42 extracted at step s 24 is set to the extension telephone set 40 at the time when the call is received . the calling id information element “ 044777 xxxx ” of the telephone set 42 is displayed on the extension telephone set 40 . hence , the telephone set 40 can make a return call . then , the extension telephone set 40 replies to the call at step s 30 , so that a mutual communication between the extension telephone set 40 and the telephone set 42 can take place . a description will be given of the case where the telephone set 42 calls the telephone set 44 , as indicated by a broken line shown in fig1 . fig1 is a sequence diagram showing how information is transferred . fig1 shows call setup messages used in the present case . at step s 10 , the telephone set 42 connected to the isdn 34 sends a call to the pbx 30 . hence , the isdn 34 sends a call setup message sm 3 shown in fig1 to the pbx 30 . the call setup message sm 3 includes “ 044777 xxxx ” as the calling id information element of code set # 0 #, and “ 044777 ****” as the called id information element . at step s 12 , the call setup message sm 3 arrives at the pbx 30 , which extracts , at step s 14 , the calling id information element and the called id information element from code set # 0 of the call setup message sm 3 . at step s 16 , the controller 54 refers to the dil translation table 65 by referring to the called id information element extracted at step s 14 , and translates it into the via - pbx 32 called id information element . in the present case , “ 044777 ****” is translated into “ 710 + 0 + 06949 yyyy ”. at step s 18 , the controller 54 identifies the connection called from the called id information . in the present case , the connection called is the isdn tie line 36 , and thus the process proceeds to step s 20 . at step s 20 , a call setup message sm 4 to be transferred over the isdn tie line 36 is assembled . in the above call setup message sm 4 , the pbx number “ 700 ” of the pbx 32 is written into the calling id information element m 1 of code set # 0 shown in fig4 . further , the call setup message sm 4 includes the prefix dial number “ 0 ” and the extracted calling id information element “ 044777 xxxx ”. further , in the call setup message sm 4 , the via - pbx 32 called id information element “ 710 + 0 + 06949 yyyy ” obtained by the translation of step s 16 is written into the called id information element of code set # 0 of the call setup message sm 4 . further , the calling id information element “ 044777 xxxx ” extracted at step s 14 is written into the calling id information element m 3 of code set # 7 . the call setup message sm 4 thus assembled is sent to the pbx 32 at step s 22 . at step s 24 , the controller 54 of the pbx 32 extracts the calling id information element “ 044777 xxxx ” of the telephone set 42 from the calling id information element of code set # 7 of the call setup message sm 4 transferred over the isdn tie line 36 , and further extracts the called id information element of code set # 0 therefrom . at step s 26 , the controller 54 identifies the connection called from the called id information element extracted at step s 24 . in the present case , the connection called is the telephone set 44 connected to the isdn 34 , and the process proceeds to step s 32 . at step s 32 , the controller 54 writes the calling id information element “ 044777 xxxx ” of the call telephone set 42 extracted at step s 24 into the calling id information element m 1 of code set # 0 shown in fig4 . further , the controller 54 deletes the pbx number and the prefix dial number “ 700 + 0 ” from the called id information element “ 710 + 0 + 06949 yyyy ” extracted ate step s 24 . then , the rest of the called id information element is written into the called id information element m 2 of code set # 0 . a call setup message sm 5 shown in fig1 and thus configured as described above is sent to the isdn 34 . at step s 34 , at the time when the call arrives at the telephone set 44 , the calling id information element “ 044777 xxxx ” is displayed on the terminal telephone set 44 , so that the telephone set 44 can reply to the telephone set by return and a communication between the terminals 42 and 44 can take place . [ 0068 ] fig1 shows a case where the extension telephone set 38 of the pbx 30 calls the telephone set 44 connected to the isdn 34 . fig1 shows an information transfer sequence in the present case . fig1 shows call setup messages in the present case . at step s 40 shown in fig1 a , the extension telephone set 38 calls the pbx 30 . at step s 42 , the controller 54 of the pbx 30 extracts an extension number dn of the extension telephone set 38 and the called id information element from a message sm 6 shown in fig1 . the called id information element includes the number “ 710 ” of the pbx 32 , the access number “ 0 ” for making a connection with the isdn 34 , and the id of the telephone set 44 . at step s 44 , the controller 54 of the pbx 30 refers to the extension dial - in number translation table 66 by using the above calling id information element , and translates it into the dial - in number of the extension telephone set 38 . in the present case , the extension number dn is translated into the dial - in number “ 044777 aaaa ”. at step s 46 , the controller 54 identifies the connection called from the called id information . in the present case , the connection called is the isdn tie line 36 , and the process proceeds to step s 48 . at step s 48 , a call setup message sm 7 ( fig1 ) to be transferred over the isdn tie line 36 is assembled . the call setup message sm 7 includes the pbx number “ 700 ” of the pbx 32 and the extension number dn of the extension telephone set 38 written into the calling id information element m 1 of code set # 0 shown in fig5 . further , the call setup message sm 7 includes the called id information element “ 710 + 0 + 06949 yyyy ” written into the called information element of code set # 0 , and the dial - in number “ 044777 aaaa ” obtained by the translation of step s 44 and written in the dial - in number m 4 of code set # 7 . at step s 50 , the call setup message sm 7 thus produced is sent to the pbx 32 . at step s 52 , the controller 54 of the pbx 32 extracts the dial - in number “ 044777 aaaa ” of code set # 7 of the call setup message sm 7 transferred over the isdn tie line 36 , and extracts the called id information element of code set # 0 . at step s 54 , the controller 54 identifies the connection called from the extracted called id information . in the present case , the connection called is the telephone set 44 connected to the isdn 34 , and thus the process proceeds to step s 56 . at step s 56 , the controller 54 writes the dial - in number “ 044777 aaaa ” of the call extension telephone set extracted at step s 52 into the calling id information element m 1 of code set # 0 shown in fig4 . further , the controller 54 deletes the pbx number and the access number “ 700 + 0 +” from the called id information element extracted at step s 52 , and writes the rest of the called id information element into the called id information element of code set # 0 . a call setup message sm 8 shown in fig1 from which code set # 7 has been deleted is sent to the isdn 34 . at step s 58 , the calling id information element given in the call setup message sm 8 is displayed on the telephone set 44 when receiving the call . hence , the telephone set 44 can reply to the call from the extension telephone set 38 by return . at step s 60 , the telephone set 44 responds to the call so that a communication between the extension telephone set 38 and the telephone set 44 can take place . [ 0074 ] fig1 shows a case where the extension telephone set 38 calls the extension telephone set 40 . fig1 shows an information transfer sequence in the case shown in fig1 . fig1 shows call setup messages used in the present case . at step s 40 , the extension telephone set 38 calls the pbx 30 . at step s 42 , the controller of the pbx 30 extracts the extension number dn of the extension telephone set 38 and the called id information element from the message sm 6 shown in fig1 . in the present case , the called id information element is “ 710 + rrrr ”. at step s 44 , the controller 54 of the pbx 30 refers to the extension dial - in number translation table 66 by using the above calling id information element , and translates it into the dial - in number of the extension telephone set 38 . in the present case , the extension number dn of the extension telephone set 38 is translated into “ 044777 aaaa ”. at step s 46 , the controller 54 identifies the connection called from the called id information element . in the present case , the connection called is the isdn tie line 36 , and thus the process proceeds to step s 48 . at step s 48 , the controller 54 assembles the call setup message sm 7 shown in fig1 to be transferred over the isdn tie line 36 . in the call setup message sm 7 , the pbx number “ 700 ” of the pbx 32 and the extension number dn of the extension telephone set 38 are written into the calling id information element m 1 of code set # 0 shown in fig5 . further , the called id information element “ 710 + rrrr ” is written into the called id information element of code set # 0 , and the dial - in number “ 044777 aaaa ” obtained by the translation of step s 44 is written into the dial - in number m 4 of the pbx extension of code set # 7 . at step s 50 , the call setup message sm 7 thus produced is sent to the pbx 32 . at step s 52 , the controller 54 of the pbx 32 extracts the dial - in number of the extension 38 from code set # 7 from the call setup message sm 7 transferred over the isdn tie line 36 , and extracts the called id information element of code set # 0 . at step s 54 , the controller 54 identifies the connection called from the called id information element extracted at step s 54 . in the present case , the connection called is the extension telephone set 40 connected to the pbx 32 , and thus the process proceeds to step s 62 . at step s 62 , the calling id information element of the extension telephone set 38 extracted at step s 52 , that is , the dial - in number thereof is set to the extension telephone set 40 when receiving the call . then , the dial - in number “ 044777 aaaa ” is displayed on the extension telephone set 40 , so that a return call to the extension telephone set 38 can be made . thereafter , at step s 64 , the extension telephone set 40 responds to the call from the extension telephone set 38 , so that a communication between the telephone sets 40 and 42 can take place . [ 0078 ] fig2 a and 20b are flowcharts of a sequence in which the extension telephone set 38 calls the extension telephone set 40 via the pbx 30 , the isdn tie line 36 and the pbx 32 , as indicated by a broken line shown in fig2 , and the above call is transferred to a telephone set 44 connected to the isdn 34 . in fig2 a and 20b , parts that are the same as those shown in fig1 a and 13b are given the same reference numbers . further , fig2 shows an information transfer sequence in the present case . at step s 40 , the extension telephone set 38 calls the pbx 30 . at step s 42 , the controller 54 of the pbx 30 extracts the extension number dn of the extension telephone set 38 and the called id information element from the message sm 6 shown in fig1 . in the present case , the called id information element is “ 710 + rrrr ”. the controller 54 of the pbx 30 refers to the extension dial - in number translation table 66 by using the calling id information element , and translates it into the dial - in number of the extension telephone set 38 . in the present case , the extension number dn is translated into “ 044777 aaaa ”. in step s 46 , the controller 54 identifies the connection called by referring to the called id information . in the present case , the connection called is the isdn tie line 36 , and thus the process proceeds with step s 48 . at step s 48 , the controller 54 assembles the call setup message sm 7 ( fig1 ) to be transferred over the isdn tie line 36 . in the call setup message sm 7 , the pbx number “ 700 ” of the pbx 32 and the extension number dn of the extension telephone set 38 are written into the calling id information element of code set # 0 shown in fig5 . the called id information element “ 710 + rrrr ” is written into the called id information element m 2 of code set # 0 . the dial - in number “ 044777 aaaa ” obtained by the translation of step s 44 is written into the dial - in number m 4 of code set # 7 . at step s 50 , the call setup message sm 7 is sent to the pbx 32 shown in fig1 . at step s 52 , the controller 54 of the pbx 32 extracts the dial - in number “ 044777 aaaa ” of code set # 7 from the call setup message sm 7 transferred over the isdn tie line 36 , and extracts the called id information element of code set # 0 . at step s 54 , the controller 54 identifies the connection called from the called id information element extracted at step s 52 . in the present case , the connection called is the extension telephone set 40 of the pbx 32 , and the process proceeds to step s 62 . at step s 62 , the controller 54 sets the calling id information element of the call extension telephone set 38 extracted at step s 52 , that is , the dial - in number thereof to the extension telephone set 40 . hence , the dial - in number “ 044777 aaaa ” is displayed on the extension telephone set 40 . at step s 64 , the extension telephone set 40 replies to the call from the telephone set 42 so that a communication therebetween can take place . when it is found , during speech communication , that the operator x of the telephone set 42 wishes to make a speech communication with the operator y of the telephone set 44 connected to the isdn 34 , at step s 70 , a call addressed to the telephone set 44 is made by the extension telephone set 40 , and the process proceeds to step s 54 . at step s 54 , the connection called is identified . in the present case , the connection called is the telephone set 44 connected to the isdn 34 , and thus the process proceeds to step s 56 . at step s 56 , the dial - in number “ 044777 aaaa ” of the extension telephone set extracted at step s 52 is written into the calling id information element m 1 of code set # 0 shown in fig4 . further , “ 06949 yyyy ” of which the operator of the extension telephone set 38 of the pbx 30 informed the operator of the extension telephone set 40 of the pbx 32 is written into the called id information element m 2 . then , the resultant call setup message sm 8 shown in fig1 from which code set # 7 has been deleted is sent to the isdn 34 . at step s 58 , the call setup message sm 8 is received by the telephone set 44 on which the calling id information element included therein , that is , the dial - in number “ 044777 aaaa ” is displayed . hence , a return call can be made . at step s 60 , the telephone set 44 replies to the call , and a communication between the telephone sets 44 and 40 can take place . [ 0085 ] fig2 a and 23b are flowcharts of a sequence in which the telephone set 42 calls the extension telephone set 38 via the isdn 34 and the pbx 30 , and the call is transferred to the extension telephone set 40 of the pbx 32 , as shown in fig2 . in fig2 a and 23b , parts that are the same as those shown in fig6 a and 6b are given the same reference numbers . fig2 shows an information transfer sequence in the present case . at step s 10 , the telephone set 42 connected to the isdn 34 calls the pbx 30 . thus , the call setup message sm 1 is sent to the pbx 30 from the isdn network 34 . in the call setup message sm 1 , “ 044777 xxxx ” is written into the calling id information element of code set # 0 , and “ 044777 efgh ” is written into the called id information element . the pbx 30 receives the call setup message at step s 12 , and extracts the calling id information element and the called id information element from code set # 0 of the call setup message sm 1 . at step s 16 , the controller 54 of the pbx 30 refers to the dil translation table 65 by using the called id information element extracted at step s 14 , and translates it into the via - pbx 32 called id information element . in the present case , the called id information element indicates the extension telephone set 38 , and thus the process proceeds to step s 80 . at step s 80 , the controller 54 sets the calling id information element to the extension telephone set 38 and causes this element to be displayed thereon when receiving the call . at step s 82 , the extension telephone set 38 replies to the call , and thus a communication between the telephone sets 42 and 38 can take place . when it is found , during speech communication , that the operator of the telephone set 42 wishes to make a speech communication with the extension telephone set 40 of the pbx 32 , at step s 84 , a call addressed to the telephone set 40 is made by the extension telephone set 38 , and the process proceeds to step s 18 . at step s 18 , the controller 54 of the pbx 32 identifies the connection called from the called id information element . in the present case , the connection called is the isdn tie line 36 , and thus the process proceeds to step s 20 . at step s 20 , the controller 54 assembles the call setup message to be transmitted over the isdn tie line 36 . in the call setup message , the pbx number “ 700 ” of the pbx 32 , the access number “ 0 ” for making a connection to the isdn 34 , and the extracted calling id information element “ 044777 xxxx ” are written into the calling id information element . further , the orally informed “ 710 + zzzz ” of the extension telephone set 38 is written into the called id information element m 2 of code set # 0 . furthermore , the calling id information element “ 044777 xxxx ” extracted at step s 14 is written into the calling id information element m 3 of code set # 7 . at step s 22 , the call setup message thus produced is sent to the pbx 32 . at step s 24 , the controller 54 of the pbx 32 extracts the calling id information element “ 044777 xxxx ” of the telephone set 42 from code set # 7 of the call setup message , and extracts the called id information element from code set # 0 . at step s 26 , the controller 54 identifies the connection called from the called id information element . in the present case , the connection called is the extension telephone set 40 of the pbx 32 , and thus the process proceeds to step s 28 . at step s 28 , the calling id information element of the call telephone set 42 extracted at step s 24 is set to the extension telephone set 40 , on which the calling id information element “ 044777 xxxx ” is displayed . hence , a return call can be made . at step s 30 , the extension telephone set 40 replies to the call , so that a speech communication between the telephone sets 42 and 40 can take place . then , the process proceeds to step s 86 , at which the extension telephone set 38 is disconnected and the telephone sets 42 and 40 are connected by the pbx 30 , so that a speech communication therebetween can take place . the present invention is not limited to the specifically disclosed embodiments , and variations and modifications may be made without departing from the scope of the present invention .