Abstract:
A switch device connectable to an ISDN includes a first part which adds a calling ID information element to a first call setup message to be transferred over an ISDN tie line connecting the switch device to another switch device. The calling ID information element includes identification information on a terminal which is connected to the ISDN and makes a call.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to a private branch exchange (PBX) network system, and more particularly to a private branch exchange network system including a plurality of private branch exchanges connected to an integrated services digital network (ISDN).  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 1 is a diagram showing a PBX network having PBXs  10  and  12  connected to an ISDN  14 . The PBXs  10  and  12  are local switch devices. An ISDN tie line  16  connects the PBXs  10  and  12  together. Terminals such as telephone sets are connected to the PBXs  10  and  12  so that a PBX network is formed. For the sake of simplicity, only one telephone set  18  connected to the PBX  10  is illustrated, and only one telephone set  20  connected to the PBX  12  is illustrated. The telephone sets  18  and  20  are respectively connected to extensions of the PBXs  10  and  12 , and will be referred to as extension telephone sets hereinafter.  
           [0005]    The PBX  10  sends a message to the PBX  12  over the ISDN tie line  16 , and PBX  12  sends a message to the PBX  10  thereover. For example, the message sent by the PBX  10  includes the identification number of the PBX  10 , and a ID of the calling terminal, that is, the extension telephone set  18 . In FIG. 1, the PBXs  10  and  12  are respectively assigned identification numbers “ 700 ” and “ 710 ”, and the extension telephone sets  18  and  20  are respectively assigned IDs “A” and “B”.  
           [0006]    Telephone sets  22  and  24  are connected to the ISDN  14 , and are assigned IDs “X” and “Y”, respectively. The telephone set  22  can call the extension telephone set  20  via the ISDN  14 , the PBX  10 , the ISDN tie line  16  and the PBX  12 . However, in this case, the message sent to the PBX  12  from the PBX  10  includes the identification number “ 700 ” of the PBX  10  and the identification number of the ISDN network, but does not include the ID “X” of the calling telephone set  22 . Hence, the called telephone set  20  is not informed of the ID “X” of the calling telephone set  22 . This is inconvenient when the extension telephone set  20  calls the telephone set  22  by return.  
           [0007]    The extension telephone set  18  can call the telephone set  24  via the ISDN tie line  16 , the PBX  12 , the ISDN  14 . In this case, the message sent to the PBX  12  from the PBX  10  includes the identification number “ 700 ” of the PBX  10  and the ID “A” of the calling telephone set  18 . However, the message sent to the telephone set  24  from the ISDN  14  does not include the ID “A” of the calling telephone set  18 . This is inconvenient when the telephone set  24  calls the extension telephone set  18  connected to the PBX  10  by return.  
         SUMMARY OF THE INVENTION  
         [0008]    It is a general object of the present invention to provide a switch device and a system in which the above disadvantages are eliminated.  
           [0009]    A more specific object of the present invention is to provide a switch device and a system capable of informing a called terminal of information indicative of the identification of the calling terminal, which may be accommodated by a local switch (PBX) or an ISDN, so that the identification information of the calling terminal can be displayed or the called terminal can easily call the calling terminal by return.  
           [0010]    These objects of the present invention are achieved by a switch device connectable to an ISDN, the switch device comprising: a first part which adds a calling ID information element to a first call setup message to be transferred over an ISDN tie line connecting the switch device to another switch device, the calling ID information element including identification information on a terminal which is connected to the ISDN and makes a call.  
           [0011]    The switch device may further comprise a second part which receives the first call setup message transferred via the ISDN tie line from another switch device and writes the calling ID information element into a second call setup message to be transferred over the ISDN so that the switch device calls the terminal connected to the ISDN.  
           [0012]    The switch device may further comprise a third part which allows a first extension terminal connected to the switch device to communicate with the terminal connected to the ISDN upon receiving a call therefrom and then calls a second extension terminal connected to another switch device or another terminal connected to the ISDN via the ISDN tie line.  
           [0013]    The above objects of the present invention are also achieved by a switch device connectable to an ISDN, the switch device comprising: a first part which translates an extension number of an extension terminal connected to the switch device into a dial-in number; and a second part which adds the dial-in number to a first call setup message to be transferred over an ISDN tie line connecting the switch device to another switch device.  
           [0014]    The switch device may further comprise a third part which receives the first call setup message transferred via the ISDN tie line from another switch device and writes the dial-in number into a second call setup message to be transferred over the ISDN so that the switch device calls the terminal connected to the ISDN.  
           [0015]    The switch device may further comprise a fourth part which allows a first extension terminal connected to the switch device to communicate with the terminal connected to the ISDN upon receiving a call therefrom and then calls a second extension terminal connected to another switch device or another terminal connected to the ISDN via the ISDN tie line.  
           [0016]    The above objects of the present invention are also achieved a system comprising: a first device and a second device which are connected to an ISDN, the first switch device comprising: a first part which adds a calling ID information element to a first call setup message to be transferred over an ISDN tie line connecting the first and second switch devices together, the calling ID information element including identification information on a terminal which is connected to the ISDN and makes a call.  
           [0017]    The system may be configured so that the second switch device comprises a second part which receives the first call setup message transferred from the first switch device via the ISDN tie line and writes the calling ID information element into a second call setup message to be transferred over the ISDN so that the second switch device calls the terminal connected to the ISDN.  
           [0018]    The system may be configured so that the first switch device comprises a third part which allows a first extension terminal connected to the first switch device to communicate with the terminal connected to the ISDN upon receiving a call therefrom and then calls a second extension terminal connected to the second switch device or another terminal connected to the ISDN via the ISDN tie line.  
           [0019]    The system may be configured so that the first switch device comprises a third part which allows a first extension terminal connected to the first switch device to communicate with the terminal connected to the ISDN upon receiving a call therefrom and then calls a second extension terminal connected to the second switch device or another terminal connected to the ISDN via the ISDN tie line.  
           [0020]    The above objects of the present invention are also achieved by a system comprising: a first switch and a second switch connected to an ISDN, the first switch device comprising: a first part which translates an extension number of an extension terminal connected to the first switch into a dial-in number; and a second part which adds the dial-in number to a first call setup message to be transferred to the second switch over an ISDN tie line connecting the first and second switch devices together.  
           [0021]    The system may be configured so that the first switch device comprises a third part which receives the first call setup message transferred over the ISDN tie line from the first switch device and writes the dial-in number into a second call setup message to be transferred over the ISDN so that the second switch device calls the terminal connected to the ISDN.  
           [0022]    The system may be configured so that the first switch device comprises a fourth part which allows a first extension terminal connected to the first switch device to communicate with the first terminal connected to the ISDN upon receiving a call therefrom and then calls a second extension terminal connected to the second switch device or another terminal connected to the ISDN via the ISDN tie line. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a block diagram of a PBX network having PBXs connected to an ISDN;  
         [0024]    [0024]FIG. 2 is a block diagram of an embodiment of the present invention;  
         [0025]    [0025]FIG. 3 is a block diagram of a PBX shown in FIG. 2;  
         [0026]    [0026]FIG. 4 is a diagram of a format of a call setup message;  
         [0027]    [0027]FIG. 5 is a diagram of a format of another call setup message;  
         [0028]    [0028]FIGS. 6A and 6B are flowcharts of a control sequence of the embodiment of the present invention;  
         [0029]    [0029]FIG. 7 is a block diagram showing a first operation of the embodiment of the present invention  
         [0030]    [0030]FIG. 8 is a sequence diagram of an information transfer during the first operation of the embodiment of the present invention;  
         [0031]    [0031]FIG. 9 is a diagram showing call setup messages used in the first operation of the embodiment of the present invention;  
         [0032]    [0032]FIG. 10 is a block diagram showing a second operation of the embodiment of the present invention;  
         [0033]    [0033]FIG. 11 is a sequence diagram of an information transfer carried out during the second operation;  
         [0034]    [0034]FIG. 12 is a diagram showing call setup messages used in the second operation;  
         [0035]    [0035]FIGS. 13A and 13B are flowcharts of another sequence of the embodiment of the present invention;  
         [0036]    [0036]FIG. 14 is a block diagram of a first operation in accordance with the sequence shown in FIGS. 13A and 13B;  
         [0037]    [0037]FIG. 15 is a sequence diagram of an information transfer carried out in accordance with the sequence shown in FIGS. 13A and 13B;  
         [0038]    [0038]FIG. 16 is a diagram showing call setup messages used in the first operation shown in FIG. 14;  
         [0039]    [0039]FIG. 17 is a block diagram of a second operation in accordance with the sequence shown in FIGS. 13A and 13B;  
         [0040]    [0040]FIG. 18 is a sequence diagram of an information transfer carried out in accordance with the sequence shown in FIGS. 13A and 13B;  
         [0041]    [0041]FIG. 19 is a diagram showing call setup messages used in the second operation shown in FIG. 17;  
         [0042]    [0042]FIGS. 20A and 20B are flowcharts of yet another sequence of the embodiment of the present invention;  
         [0043]    [0043]FIG. 21 is a block diagram of a first operation in accordance with the sequence shown in FIGS. 20A and 20B;  
         [0044]    [0044]FIG. 22 is a sequence diagram of an information transfer carried out during the first operation shown in FIG. 21;  
         [0045]    [0045]FIGS. 23A and 23B are flowcharts of a further sequence of the embodiment of the present invention;  
         [0046]    [0046]FIG. 24 is a block diagram of an operation in accordance with the sequence shown in FIGS. 23A and 23B; and  
         [0047]    [0047]FIG. 25 is a sequence diagram of an information transfer carried out during the operation shown in FIG. 24. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0048]    [0048]FIG. 2 shows switch devices and a system using these switches according to an embodiment of the present invention. Referring to FIG. 2, 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]    [0049]FIG. 3 is a block diagram of each of the PBXs  30  and  32  shown in FIG. 2. 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 .  
         [0050]    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 .  
         [0051]    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]    [0052]FIGS. 4 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 FIG. 4 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.  
         [0053]    Code set # 0  of the call setup message shown in FIG. 5 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.  
         [0054]    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 .  
         [0055]    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]    [0056]FIGS. 6A 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.  
         [0057]    First, the case where a call is addressed to the extension telephone set  40  will be described with reference to FIG. 7, in which a call is transferred as indicated by the broken line. FIG. 8 will also be referred to in order to explain how information is transferred. Step numbers shown in FIG. 7 correspond to those shown in FIG. 8. Further, FIG. 9 which explains the call setup message will be referred to.  
         [0058]    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 FIG. 9 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 .  
         [0059]    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”.  
         [0060]    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 .  
         [0061]    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.  
         [0062]    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 FIG. 10. FIG. 11 is a sequence diagram showing how information is transferred. FIG. 12 shows call setup messages used in the present case.  
         [0063]    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 FIG. 12 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.  
         [0064]    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 FIG. 4. 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 .  
         [0065]    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 .  
         [0066]    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 FIG. 4. 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 FIG. 12 and thus configured as described above is sent to the ISDN  34 .  
         [0067]    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]    [0068]FIG. 14 shows a case where the extension telephone set  38  of the PBX  30  calls the telephone set  44  connected to the ISDN  34 . FIG. 15 shows an information transfer sequence in the present case. FIG. 16 shows call setup messages in the present case.  
         [0069]    At step S 40  shown in FIG. 13A, 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 FIG. 16. 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 .  
         [0070]    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  (FIG. 16) 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 FIG. 5. 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 .  
         [0071]    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 .  
         [0072]    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 FIG. 4. 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 FIG. 16 from which code set # 7  has been deleted is sent to the ISDN  34 .  
         [0073]    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]    [0074]FIG. 17 shows a case where the extension telephone set  38  calls the extension telephone set  40 . FIG. 18 shows an information transfer sequence in the case shown in FIG. 17. FIG. 19 shows call setup messages used in the present case.  
         [0075]    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 FIG. 19. In the present case, the called ID information element is “ 710 +RRRR”.  
         [0076]    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 FIG. 19 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 FIG. 5. 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 .  
         [0077]    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]    [0078]FIGS. 20A 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 FIG. 21, and the above call is transferred to a telephone set  44  connected to the ISDN  34 . In FIGS. 20A and 20B, parts that are the same as those shown in FIGS. 13A and 13B are given the same reference numbers. Further, FIG. 22 shows an information transfer sequence in the present case.  
         [0079]    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 FIG. 19. In the present case, the called ID information element is “ 710 +RRRR”.  
         [0080]    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  (FIG. 19) 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 FIG. 5. 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 FIG. 19.  
         [0081]    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.  
         [0082]    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 .  
         [0083]    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 FIG. 4. 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 FIG. 16 from which code set # 7  has been deleted is sent to the ISDN  34 .  
         [0084]    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]    [0085]FIGS. 23A 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 FIG. 24. In FIGS. 23A and 23B, parts that are the same as those shown in FIGS. 6A and 6B are given the same reference numbers. FIG. 25 shows an information transfer sequence in the present case.  
         [0086]    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.  
         [0087]    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 .  
         [0088]    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 .  
         [0089]    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.  
         [0090]    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.  
         [0091]    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.