Control method for cordless telephone which displays identities of responding called parties
This is a continuation of application Ser. No. 07/380,360 filed Jul. 17, 1989 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a control method for a cordless telephone system for connecting a plurality of stationary apparatuses and a plurality of movable portable telephones through radio channels and, more particularly, to an extension call among the portable telephones. 
     FIG. 1 shows an arrangement of a cordless telephone system. In FIG. 1, reference numeral 1 denotes a main apparatus connected to a plurality of telephone lines and having a line connection function; 10 l  to 10 n , stationary apparatuses each having a radio transmitter/receiver; 20 l  to 20 m , portable telephones; and 30 l  to 30 n , radio zones within which the portable telephones 20 l  to 20 m  can perform communication. 
     The radio zones 30 l  to 30 n  respectively correspond to the stationary apparatuses 10 l  to 10 n . The portable telephones 20 l  to 20 m  can be connected to a corresponding one of the stationary apparatuses from anywhere within the assigned radio zones 30 l  to 30 n , and can perform an office line outgoing/incoming call, an extension outgoing/incoming call, and the like. 
     In the cordless telephone system with the above arrangement, however, although one portable telephone can call all other portable telephones, the calling portable telephone cannot detect whether or not other portable telephones perform an incoming call display. This will be exemplified below. 
     FIG. 2 shows a conventional control method of a cordless telephone. In this case, connection procedures of simultaneous calling are illustrated, i.e., when a caller in just a single calling operation calls multiple destinations. For the sake of simplicity, the system is assumed to be constituted by a main apparatus, first to third stationary apparatuses, and first to third portable telephones. Assume that the first to third telephones belong to radio zones of the first to third stationary apparatuses, respectively. Furthermore, the second portable telephone is connected to the second stationary apparatus through a radio channel and is busy through an office line. 
     In FIG. 2, the first portable telephone sends a call request signal S 21  to the first stationary apparatus in response to a simultaneous calling operation by a user. Upon reception of the call request signal S 21 , the first stationary apparatus transmits a call request signal S 22  to the main apparatus. When the main apparatus receives the call request signal S 22 , it transmits an incoming call signal S 23  to the third stationary apparatus which is not in use (the second stationary apparatus is in use). Upon reception of the incoming call signal S 23 , the third stationary apparatus transmits an incoming call signal S 24  to the third portable telephone. The third portable telephone performs an incoming call display upon reception of the incoming call signal S 24 , and informs a user of the third portable telephone of simultaneous calling. 
     However, although the first portable telephone performs simultaneous calling to the second and third portable telephones, the second portable telephone (not shown) is busy through an office line, and only the third portable telephone performs an incoming call display. Meanwhile, the first portable telephone cannot detect whether or not other portable telephones perform the incoming call display. Thus, a user of the first portable telephone makes a wrong judgement that the second portable telephone performs the incoming call display. 
     SUMMARY OF THE INVENTION 
     It is therefore a principal object of the present invention to provide a control method for a cordless telephone system, in which a response signal including an extension number is transmitted to a portable telephone requesting simultaneous calling, so that extension numbers of portable telephones which respond to simultaneous calling can be detected. 
     In order to achieve the above object, according to an aspect of the present invention, there is provided a control method for a cordless telephone system which comprises: a main apparatus connected to a telephone line and having a line control function; a plurality of stationary apparatuses connected to the main apparatus and each having a radio transmitter/receiver; and portable telephones capable of communicating with the stationary apparatuses through a radio channel, wherein when the main apparatus receives a simultaneous calling request signal from one portable telephone through a given stationary apparatus, the main apparatus transmits a simultaneous calling request signal to the other portable telephone which is not in use on the basis of the received simultaneous calling request signal through a corresponding stationary apparatus, and receives, from the other portable telephone through the corresponding stationary apparatus, a response signal including an extension number of the other portable telephone which received the simultaneous calling request signal, and transmits a response signal including the extension number of the other portable telephone which responded to simultaneous calling to the one portable telephone which requested simultaneous calling through the given stationary apparatus, and the one portable telephone which requested simultaneous calling displays the extension number of the other portable telephone which responded to the simultaneous calling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing an arrangement of a cordless telephone system; 
     FIG. 2 is a chart showing a conventional control method for a cordless telephone system; and 
     FIG. 3 is a chart showing a control method for a cordless telephone system according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 3 shows a control method of a cordless telephone system according to the present invention. For the sake of simplicity, assume that this embodiment comprises first to third stationary apparatuses and first to third portable telephones, and that the first to third portable telephones belong to radio zones of the first to third stationary apparatuses, respectively. In addition, assume that the second portable telephone is connected to the second stationary apparatus through a radio channel and is busy through an office line. 
     In FIG. 3, the first portable telephone transmits a simultaneous calling request signal S 1  to the first stationary apparatus in response to a simultaneous calling operation by a user. The first stationary apparatus transmits a simultaneous calling request signal S 2  to the main apparatus upon reception of the simultaneous calling request signal S 1 . When the main apparatus receives the simultaneous calling request signal S 2 , it transmits an incoming call signal S 3  to the third stationary apparatus which is not in use (the second stationary apparatus is in use). Upon reception of the incoming call signal S 3 , the third stationary apparatus transmits an incoming call signal S 4  to the third portable telephone which belongs to it. The third portable telephone performs an incoming call display upon reception of the incoming call signal S 4  to inform a user of the simultaneous calling. The third portable telephone transmits an incoming call response signal S 5  including its own extension number to the third stationary apparatus. Upon reception of the response signal S 5 , the third stationary apparatus transmits an incoming call signal S 6  including the extension number of the portable telephone which responded to the simultaneous calling to the main apparatus. 
     When the main apparatus receives the incoming call response signal S 6 , it transmits a response display signal S 7  including the extension number of the portable telephone which responded to the simultaneous calling to the first stationary apparatus which requested the simultaneous calling. Upon reception of the response display signal S 7 , the first stationary apparatus transmits a response display signal S 8  including the extension number of the portable telephone which responded to the simultaneous calling to the first portable telephone. When the first portable telephone receives the response display signal S 8 , it displays the extension number included in the input signal on a display unit. Thus, the user can know response of the third portable telephone and non-response of the second portable telephone. 
     If a large number of portable telephones are arranged, extension numbers may be displayed simultaneously, in units of zones, or in turn in accordance with the number of digits of the extension number. 
     According to the present invention, a portable telephone which is simultaneously called transmits a response signal including an extension number to a portable telephone, which requested simultaneous calling, through a stationary apparatus, the main apparatus, and another stationary apparatus, so that a user can know an extension number of a portable telephone which responded to simultaneous calling.