Abstract:
A route selection method includes the steps of hierarchically dividing voice gateways, allocating necessary information for a route selection to the respective voice gateways, controlling an originating voice gateway to transmit a destination inquiring message to a destination voice gateway via other gateways, and controlling the destination voice gateway to transmit a destination determination message to the originating voice gateway.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to voice gateways for setting up a call via an IP (Internet Protocol) network, and more particularly to a route selection method of the voice gateways for determining a destination for setting up the call. 
   2. Description of the Related Art 
   Conventionally, a call system via the IP network or the like can reduce communication costs by effectively making use of network bands. For this reason, a demand for utilizing long distance calls and the like of the call system via the IP network is high. 
     FIG. 1  illustrates an example of a conventional route selection method. 
   In this diagram, a telephone switching network  10  and an IP network  12  are connected via voice gateways  20   1  to  20   n , each having an office number indicating a location in the telephone switching network  10  and a transport address indicating a location in the IP network  12 . Telephone terminals  50   1  to  50   p  connected to the telephone switching network  10  or telephone terminals  40   1  to  40   n  respectively connected to the voice gateways  20   1  to  20   n  can communicate with each other via the telephone switching network  10  or the IP network  12 . An address determination server  30  will be described later. 
   In a case where the telephone terminal  40   1  calls the telephone terminal  40   m  via the IP network  12 , the telephone terminal  40   1  sends to the voice gateway  20   1  a telephone number of the telephone terminal  40   m  as a destination, and the voice gateway  20   1  then determines a transport address after receiving the telephone number of the telephone terminal  40   m . The transport address indicates a location of the voice gateway  20   n  as a destination for setting up the call in the IP network  12 . Thus, the voice gateway  20   1  sends out a call-setup signal towards the transport address, and if the call is set up, then a communication (talk) is started between the telephone terminal  40   1  and the telephone terminal  40   m . 
   Conventionally, there are two methods for the voice gateway  20   1  to determine the transport address of the destination voice gateway  20   n  for setting up the call according to the telephone number of the destination telephone terminal  40   m . 
   In the first method, the voice gateways  20   1  to  20   n  each hold a table in which are recorded office numbers and corresponding transport addresses of all of the voice gateways  20   1  to  20   n  existing in the IP network  12 . In the call setup, the transport address of the destination voice gateway is determined by referring to the table. 
   In the second method, the IP network is provided with the address determination server  30  having the table described in the first method. In the call setup, the transport address of the destination voice gateway is determined by inquiring of the address determination server  30 . 
   However, the above-described two methods have the following disadvantages. 
   With respect to the first method, each voice gateway has the table in which are recorded the office numbers and the corresponding transport addresses of all of the voice gateways  20   1  to  20   n  existing in the IP network  12 . For this reason, when an office number or a transport address of one voice gateway is changed due to the voice gateway being moved, the tables of all the other voice gateways must be changed. This brings about a problem of maintenance management. 
   With respect to the second method, whenever a call is generated, traffic is generated between the respective voice gateways  20   1  to  20   n  and the address determination server  30 . For this reason, a load on the address determination server  30  is too heavy. That is, inquiries from a plurality of the voice gateways are concentrated on the one address determination server  30 . This brings about the address determination server  30  operating late or congestion occurring at a point of connecting the address determination server  30  and the IP network  12 . Moreover, even if a plurality of such address determination servers are disposed to disperse the traffic, as a result, costs go up and the problems of the above-described first method occur. 
   SUMMARY OF THE INVENTION 
   It is a general object of the present invention to provide voice gateways, a route selection method of the voice gateways, and a memory medium for storing control programs of the voice gateways, in which the above disadvantages are eliminated. 
   A more specific object of the present invention is to provide voice gateways, a route selection method of, the voice gateways, and a memory medium for storing, control programs of the voice gateways, in which, when performing a route selection for setting up a call, each of the voice gateways can avoid traffic congestion by transmitting information about the route selection to determine a route, and can make maintenance management easy by automatically updating the information about the route selection. 
   The above objects of the present invention are achieved by a route selection method for a voice system, which system comprises: 
   a plurality of networks including IP networks; and 
   a plurality of voice gateways interconnecting said IP networks with the other networks for voice communication, 
   the voice gateways being hierarchically divided into first voice gateways for accommodating nodes of the networks other than the IP networks, and second voice gateways for delivering information about the route selection; 
   the method comprising the steps of: 
   (a) controlling the second voice gateways to hold a route selection table indicating a correspondence between steering numbers and transport addresses for identifying routes on the IP network; 
   (b) controlling one of the first voice gateways as an originating voice gateway to transmit a destination inquiring message, including a transport address of the originating voice gateway and a destination office number, to a predetermined one of the second voice gateways in which the destination inquiring message is collated with the route selection table thereof and is forwarded towards one of the first voice gateways as a destination voice gateway; and 
   (c) controlling the destination voice gateway to transmit a destination determination message including a transport address of the destination voice gateway to the originating voice gateway. 
   The above described step (a) may further includes a step of controlling the voice gateway, in a predetermined case, to transmit a steering number and a transport address thereof to the second voice gateway. 
   The above route selection method may further comprise a step of: 
   (d) controlling the originating voice gateway, after receiving the destination determination message, to record a destination transport address and a destination steering number, which are included in the destination determination message, in a route selection table thereof, the route selection table indicating a correspondence between the destination transport address and the destination steering number. 
   The above route selection method may further comprise a step of: 
   (e) controlling the originating voice gateway, after receiving the destination determination message, to refer to traffic between the originating voice gateway and the destination voice gateway, and then only when the traffic meets a predetermined traffic requirement, record a destination transport address and a destination steering number which are included in the destination determination message in a route selection table thereof, the route selection table indicating a correspondence between the destination transport address and the destination steering number. 
   The above route selection method may further comprise a step of: 
   (f) controlling the originating voice gateway, in a case of failing to set up a call towards the transport address recorded in the route selection table thereof, to transmit the destination inquiring message to the predetermined second voice gateway. 
   The above route selection method may further comprise a step of: 
   (g) deleting the transport address and the steering number from the route selection table, if the transport address recorded on the route selection table is not referred to during a predetermined period. 
   The above route selection method may further comprise a step of: 
   (h) controlling the second voice gateway, in a case of failing to deliver the route selection information, to transmit a failure message to the originating voice gateway, so that the originating voice gateway selects the network other than the IP networks to set up the call. 
   The above route selection method may further comprise a step of: 
   (i) notifying the second voice gateway, when the steering number or the transport address of the voice gateway is changed, of the changed steering number or the changed transport address. 
   The above objects of the present invention can be achieved by a voice gateway interconnecting IP networks with other networks for voice communication, the voice gateway comprising: 
   a call-setup part which, after detecting a call-setup request from one of the other networks, transmits a destination inquiring message to a predetermined voice gateway, and, after receiving a destination determination message from a destination voice gateway performs a call setup towards a destination transport address included in the destination determination message from the destination voice gateway. 
   The above voice gateway may further comprise a transmitting part which, in a predetermined case, transmits a steering number and a transport address of the voice gateway to the predetermined voice gateway. 
   The above voice gateway may further comprise a recording part which records the destination transport address and a destination steering number of the destination voice gateway, which are included in the destination determination message, in a route selection table thereof, the route selection table indicating a correspondence between the destination transport address and the destination steering number of the destination voice gateway. 
   The above voice gateway may further comprise a recording part which, after receiving the destination determination message from the destination voice gateway, refers to traffic between the voice gateway and the destination voice gateway, and, only when the traffic meets a predetermined traffic requirement, records the destination transport address and a destination steering number of the destination voice gateway in a route selection table thereof, the route selection table indicating a correspondence between the destination transport address and the destination steering number of the destination voice gateway. 
   The above voice gateway may be configured such that the call-setup part comprises: 
   a quick-call-setup part, which after detecting the call-setup request, refers to the route selection table so as to obtain the destination transport address, and then performs the call setup towards the destination transport address; and 
   a transmitting part which, in a case of the quick-call-setup part failing to perform the call setup towards the destination transport address, transmits the destination inquiring message to the predetermined voice gateway. 
   The above voice gateway may further comprise a deleting part which deletes the destination transport address and the corresponding destination steering number from the route selection table if the destination transport address recorded on the route selection table is not referred to during a predetermined period. 
   The above voice gateway may further comprise: 
   a receiving part which receives a failure message in a case where the destination inquiring message has been transmitted to the predetermined voice gateway but the destination cannot be detected; and 
   an alternate-call-setup part which selects a network other than one of the IP networks to perform the call setup when the failure message is received. 
   The above voice gateway may further comprise a notifying part which, when the steering number and the transport address of the voice gateway are changed, notifies the predetermined voice gateway of the changed steering number and the changed transport address. 
   The above objects of the present invention can be achieved by a voice gateway interconnecting IP networks and other networks for voice communication, the voice gateway comprising: 
   a transmitting part which, after receiving a destination inquiring message, refers to a route selection table thereof, which route selection table indicates a correspondence between steering numbers and transport addresses, and then transmits the destination inquiring message towards a transport address obtained by referring to the route selection table. 
   The above voice gateway may further comprise: 
   a transmitting part which, in a predetermined case, transmits a steering number and a transport address to a predetermined voice gateway; and 
   a recording part which, after receiving a steering number and a transport address from another voice gateway in the predetermined case, records the received steering number and the received transport address in the route selection table thereof. 
   The above voice gateway may further comprise a deleting part which deletes the transport address and the corresponding steering number from the route selection table if the transport address recorded in the route selection table is not referred to during a predetermined period. 
   The above voice gateway may further comprise a failure-message transmitting part which transmits a failure message to an originating voice gateway which has transmitted the destination inquiring message, in a case where the destination cannot be detected even though the destination inquiring message has been received and the route selection table has been referred to. 
   The above voice gateway may further comprise: 
   a notifying part which, when the steering number and the transport address are changed, notifies a predetermined voice gateway of the changed steering number and the changed transport address; and 
   a recording part which receives a steering number and a transport address from another voice gateway and records the steering number and the transport address in the route selection table. 
   The above objects of the present invention can be achieved by a computer readable medium storing program code for controlling a voice gateway interconnecting IP networks and other networks for voice communication, the computer readable medium comprising: 
   program code means for, after the voice gateway detects a call-setup request from one of the other networks, controlling the voice gateway to transmit a destination inquiring message to a predetermined voice gateway, and after the voice gateway receives a destination determination message, controlling the voice gateway to perform a call setup towards a destination transport address included in the destination determination message. 
   The above objects of the present invention can be achieved by a computer readable medium storing program code for controlling a voice gateway interconnecting IP networks and other networks for voice communication, the computer readable medium comprising: 
   program code means for, after the voice gateway receives a destination inquiring message, controlling the voice gateway to refer to a route selection table indicating a correspondence between steering numbers and transport addresses, and then transmit the destination inquiring message towards a transport address obtained by referring to the route selection table. 
   The above objects of the present invention can be achieved by a route selection method for voice gateways which are hierarchically divided into first voice gateways for accommodating nodes of networks other than IP networks, and second voice gateways for transferring information about a route selection, the method comprising the steps of: 
   (a) controlling each of the second voice gateways to hold a route selection table indicating a correspondence between steering numbers and transport addresses for identifying routes on the IP network; 
   (b) controlling one of the first voice gateways as an originating voice gateway to transmit a destination inquiring message, including a transport address of the originating voice gateway and a destination office number, to a predetermined one of the second voice gateways in which the destination inquiring message is collated with the route selection table thereof and is forwarded towards one of the first voice gateways as a destination voice gateway; and 
   (c) controlling the destination voice gateway to transmit a destination determination message including a transport address of the destination voice gateway to the originating voice gateway. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a network diagram illustrating a conventional route selection method of voice gateways; 
       FIG. 2  is a network diagram illustrating a route selection method of voice gateways according to an embodiment of the present invention; 
       FIG. 3  shows declaration tables (a), (b), and (c) of the voice gateways according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 4  shows route selection tables (a) and (b) of parent voice gateways according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 5  is a network diagram illustrating how a message for a call setup flows according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 6  shows a declaration message transmitted by a child voice gateway  64  according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 7  shows a route selection table of the child voice gateway  64  after transmitting a destination inquiring message toward a telephone terminal  76  according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 8  shows route selection tables (a), (b) and (c), each including a last access field, of the voice gateways  60 ,  62 , and  64  according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 9  is a network diagram illustrating how a message for performing a call setup flows in a case of failing to detect a destination according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 10  is a block diagram of the child voice gateway according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 11  is a block diagram of the parent voice gateway according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 12  is a flowchart of operations of the child voice gateway to transmit a destination inquiring message according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 13  is a flowchart of operations of the child voice gateway to receive the destination inquiring message according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 14  is a flowchart of operations of the child voice gateway in a case of performing a call setup by using the route selection table thereof according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 15  is a flowchart of operations of the child voice gateway in a case of performing a call setup by using the route selection table, in a case of referring to traffic, according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 16  is a flowchart of operations of the child voice gateway in a case of failing to set up a call according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 17  is a flowchart of operations of the child voice gateway in a case of recording a date and time on a last access field of the route selection table according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 18  is a flowchart of operations of the child voice gateway in a case of failing to set up a call via an IP network and starting the call setup again via a telephone switching network according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 19  shows tables (a) and (b) of the child voice gateway  64 , each including an update flag, according to the embodiment of the present invention shown by  FIG. 2 ; 
       FIG. 20  is a flowchart of operations of the parent voice gateway to receive the destination inquiring message according to the embodiment of the present invention shown by  FIG. 2 ; and 
       FIG. 21  is a flowchart of operations of the parent voice gateway in a case of failing to transmit the destination inquiring message according to the embodiment of the present invention shown by FIG.  2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2  is a diagram of a network consisting of a telephone switching network  10  and an IP network  12 . 
   In this diagram, the telephone switching network  10  is provided with two telephone switching offices  70  and  72  to which telephone terminals  74  and  76  are respectively connected. The IP network  12  is provided with voice gateways  60 ,  62 ,  64  and  66 , each having a conversion function of transferring voices between the telephone switching network. 10  and the IP network  12 . The voice gateways  64  and  66  are respectively connected to the telephone switching offices  70  and  72 . 
   Herein, the voice gateways  64  and  66  which are respectively connected to the telephone switching offices  70  and  72  are referred to as child voice gateways, and the voice gateways  60  and  62  which are not connected to any telephone switching offices are referred to as parent voice gateways. Further, in the claims, the child voice gateways  64 ,  66  correspond to first voice gateways and the parent voice gateways  60 ,  62  correspond to second voice gateways. 
   The switching office  70  is allocated an office number “7220” and the telephone terminal  74  is allocated a telephone number “7220-3000”. The switching office  72  is allocated an office number “7720” and the telephone terminal  76  is allocated a telephone number “7720-2000”. Each of the previously described voice gateways is allocated a steering number (digits) and an IP address. 
   Herein, the steering number is a predetermined number which represents a part of the office number of the switching office and is used to transfer a destination inquiring message. The IP address indicates a location in the IP network  12 . 
   In this embodiment, the child voice gateway  64  is allocated a steering number  722  and an IP address 20. 20. 20. 2. The child voice gateway  66  is allocated a steering number  772  and an IP address 70. 70. 70. 2. The parent voice gateway  60  is allocated a steering number  72  and an IP address 20. 20. 20. 1. The parent voice gateway  62  Is allocated a steering number  77  and an IP address 70. 70. 70. 1. 
   The voice gateways  60 ,  62 ,  64  and  66 , as shown in  FIG. 3 , have their respective declaration tables (a), (b), (c) and (d) which are respectively stored in memory units of these gateways. The declaration table (a) of the parent voice gateway  60  consists of an IP address and a port number for identifying an application, and indicates a location to which the parent voice gateway  60  transmits information about a route selection in a predetermined case (to be described later). The tables (b), (c) and (d) of the other voice gateways  62 ,  64  and  68  have the same structure as the table (a) of the voice gateway  60 , and a description thereof is omitted. 
   Hereinafter, the IP address and the port number are combined into one transport address. Further, the parent voice gateways  60  and  62 , as shown in  FIG. 4 , have their respective route selection tables (a) and (b), each including steering numbers and the transport addresses respectively corresponding to the steering numbers. 
   A description will now be given of a route selection method of the voice gateways in a case where the telephone terminal  74  calls the telephone terminal  76 , by referring to FIG.  5 . 
   A network of  FIG. 5  has the identical construction with that of FIG.  2 . When the telephone terminal  74  dials the telephone number “77202000” of the telephone terminal  76 , then the switching office  70  transmits a start signal and the number “77202000” to the child voice gateway  64 . 
   After receiving the start signal and the number “77202000” (step S 1 ), the child voice gateway  64  transmits a destination inquiring message (see  FIG. 5 ) to an IP address 20. 20. 20. 1 recorded in the declaration table (c) of the child voice gateway  64  (see FIG.  3 ), that is, to the parent voice gateway  60  (step S 2 ). As shown in  FIG. 5 , the destination inquiring message consists of a destination inquiring header, a destination number, and a transport address of the inquiring source (the child voice gateway  64 ). 
   After receiving the destination inquiring message (step S 2 ), the parent voice gateway  60  collates the destination number “77202000” with the steering number of the route selection table (a) of FIG.  4 . Since the two head digits “77” of the destination number “77202000” are consistent with the steering number “77” of the route selection table (a), the transport address (IP address: 70. 70. 70. 1, port number: 8001) is selected. Hence, the parent voice gateway  60  transmits the destination inquiring message to the selected transport address, namely, the parent voice gateway  62 . 
   After receiving the destination inquiring message (step S 3 ), the parent voice gateway  62  collates the destination number “7202000” of the destination inquiring message with the steering number of the route selection table (b) of FIG.  4 . Since the three head digits “772” of the destination number “77202000” are consistent with the steering number “772” of the route selection table (b), the transport address (IP address: 70. 70. 70. 2, port number: 8001) is selected. Hence, the parent voice gateway  62  transmits the destination inquiring message to the selected transport address, namely, the child voice gateway  66  (step S 4 ). 
   After receiving the destination inquiring message (step S 4 ), the child voice gateway  66  confirms that the steering number “772” thereof is consistent with the three head digits “772” of the destination number “77202000” of the destination inquiring message, and then transmits a destination determination message to the source transport address (IP address: 20. 20. 20. 2, port number: 8002) as shown in FIG.  5 ), that is, back to the child voice gateway  64  (step S 5 ). The destination determination message of  FIG. 5  consists of an destination determination header, a destination steering number: 772, and a destination transport address (IP address: 70. 70. 70. 2, port number: 1720). Herein, the port number  1720  denotes an application. 
   After receiving the destination determination message (step S 5 ), the child voice gateway  64  sets up a call towards the destination transport address (IP address: 70. 70. 70. 2, port number: 1720) included in the destination determination message, that is, towards the child voice gateway  66  (step S 6 ). 
   Now, a description will be given of how the route selection tables of the parent voice gateways are developed. 
   Each voice gateway, in a case of being newly disposed or being system-reset (the above-mentioned predetermined case), refers to the declaration table thereof shown in  FIG. 3 , and then transmits a declaration message (including a declaration header, a transmission source steering number, and a transmission source transport address) to a declaration destination transport address. After receiving the declaration message, each parent voice gateway records data of the declaration message in the route selection table thereof. 
     FIG. 6  shows an example of delivering the above-described declaration message. In a case of being newly disposed or being system-reset, the child voice gateway  64  refers to the declaration table (c) shown in FIG.  3  and then transmits a declaration message (including the declaration header, the transmission source steering number  722 , and the transmission source transport address (IP address: 20. 20. 20. 2, port number: 8001) to the transport address (IP address: 20. 20. 20. 1, port number: 8001), that is, to the parent voice gateway  60  (step S 7  of FIG.  5 ). After receiving the declaration message (step S 7 ), the parent voice gateway  60  records the declaration message in the route selection table (a) thereof shown in FIG.  4 . 
   Next, a description will be given of how the route selection tables of the child voice gateways are developed according to the previously described embodiment. 
   After receiving the destination determination message (step S 5  of FIG.  5 ), the child voice gateway  64  records the destination steering number  772  and the destination transport address (IP address: 70. 70. 70. 2, port number: 1720), which are included in the destination determination message, in the route selection table stored in the memory unit thereof.  FIG. 7  shows the route selection table of the child voice gateway  64 . 
   It should be noted that only in a case where traffic between the destination child voice gateway  66  and the source child voice gateway  64  is heavier than a predetermined value is the method of recording the information in the route selection table employed. 
   A description will now be given of a case where the child voice gateway  64  has been supplied with the route selection table, by referring to FIG.  5 . 
   When the telephone terminal  74  dials the telephone number “77202000” of the telephone terminal  76 , the switching office  70  transmits a start signal to the child voice gateway  64  (step S 1 ). After receiving the start signal and the dial number “77202000”, the child voice gateway  64  refers to the route selection table shown in FIG.  7  and then sets up a call towards the transport address (IP address: 70. 70. 70. 2, port number: 1720) corresponding to the steering number  772 . In this case, the steps (S 2  to S 5 ) of inquiring the destination are not carried out. 
   According to the above, a description will now be given of a case where the steering number “772” of the route selection table shown in  FIG. 7  cannot be detected. 
   If the dial number is not “77202000” but is “77502000”, then the child voice gateway  64  cannot detect the steering number “772” of the route selection table of FIG.  7 . In this case, the call setup is carried out in an order of steps S 2  to S 6 . 
   In a case where the transport address of the child voice gateway  66  is changed due to the child voice gateway  66  being moved, the child voice gateway  64  receives the start signal and the dial number “77202000” (step S 1 ), and then refers to the route selection table of  FIG. 7  so as to start a call setup. However, since the destination transport address (IP address: 70. 70. 70. 2, port number: 1720) is no longer there, the child voice gateway  64  stops setting up the call and then transmits the destination inquiring message to the parent voice gateway  60 , so that the call setup is performed according to the steps of S 2  to S 6 . 
   Also, if the transport addresses respectively included in the route selection tables of the child voice gateways and the parent voice gateways are not referred to during a predetermined period, the transport addresses and the corresponding steering numbers are automatically deleted from the route selection tables. Hence, the respective route selection tables are kept from holding needless data. For example, if the transport address (IP address: 20. 20. 20. 2, port number: 8001) of the route selection table (a) of the parent voice gateway  60  shown in  FIG. 4  is not referred to during the predetermined period, then the steering number  722  and the transport address (IP address: 20. 20. 20. 2, port number: 8001) are deleted from the route selection table (a). 
   A more detailed description thereof will now be given as follows. 
   Referring to  FIG. 5 , in a case where the telephone terminal  74  dials “77202000” at a date and time of “a” year, “b” month, “c” day, “d” hour, “e” minute and “f” second, at this date and time, after detecting the transport address (IP address: 70. 70. 70. 1, port number: 8001) according to a coincidence of the steering number “77” with the two head digits “77” of the dialed number “77202000”, the parent voice gateway  60  records a timestamp indicating the detection date and time on a last access field of a table (a) shown in FIG.  8 . Similarly, in a table (b) of  FIG. 8  the parent voice gateway  62  records such a timestamp on a last access field corresponding to the transport address (IP address: 70. 70. 70. 2, port number: 8001). 
   An example with respect to the child voice gateway will now be given as follows. 
   In  FIG. 2 , in a case where the telephone terminal  74  dials the number “77202000” at a date and time of “a” year, “b” month, “c” day, “d” hour, “e” minute and “f” second, at this date and time, after detecting the transport address (IP address: 70. 70. 70. 2, port number: 1720) according to a coincidence of the steering number “772” with the three head digits “772” of the dialed number “77202000”, the child voice gateway  64  records the timestamp indicating the detection date and time on a last access field of a table (c) shown in FIG.  8 . 
   Additionally, a description will now be given of a case where the parent voice gateway does not detect a transport address of the destination in the route selection table when transferring the destination inquiring message, by referring to FIG.  9  . 
   A constitution shown by  FIG. 9  is provided with a switching office  78  and a telephone terminal  79  connected to the switching office  78  in addition to the constitution shown by FIG.  2 . The switching office  78  is allocated an office number “7880” and the telephone terminal  79  is allocated a telephone number “7880-5000”. 
   In  FIG. 9 , after the telephone terminal  74  dials the number “78805000” of the telephone terminal  79 , the switching office  70  analyzes the dial number “78805000” so as to determine a route to the child voice gateway  64 , and then sends a start signal and the dial number “78805000” to the child voice gateway  64  (step S 11 ). After receiving the start signal and the dial number “78805000” (step S 11 ), the child voice gateway  64  transmits a destination inquiring message to the parent voice gateway  60 , information of the parent voice gateway  60  being recorded on the declaration table (c) of the child voice gateway  64  shown in  FIG. 3  (step S 12 ). The parent voice gateway  60  refers to the route selection table (a) of  FIG. 4  so as to detect a transport address corresponding to the dial number “78805000”. However, since such a transport address does not exist in the route selection table (a) of  FIG. 4 , the parent voice gateway  60  transmits a failure destination inquiring message, including only a destination inquiring header, to the child voice gateway  64  as the transmission source of the destination inquiring message (step S 13 ). 
   After receiving the failure destination inquiring message, the child voice gateway  64  starts a call setup again towards the telephone switching network  10  (step S 14 ). 
   Next, a description will be given of a case where the transport address or the steering number of each voice gateway is changed. 
   For example, when the steering number stored in the memory unit of the child voice gateway  64  is changed by a system operator from “722” to “721”, the child voice gateway  64  refers to the declaration table (c) shown in FIG.  3  and then transmits a declaration message (including the declaration header, the steering number  721 , and the transport address (IP address: 20. 20. 20. 2, port number: 8001)) to the declaration transport address (IP address: 20. 20. 20. 1, port number: 8001). The transmission route is the same as that of S 7  shown in FIG.  5 . After receiving the declaration message, the parent voice gateway  60  changes the steering number “722” of the route selection table (a) shown in  FIG. 4  into “721”. Similarly, in a case where the transport address or the steering number of each of the other voice gateways is changed, such a declaration message is transmitted to the parent voice gateway recorded in the declaration table. 
   Next, a description will be given of constitutions and operations of the voice gateways  64  and  66  in the previously described embodiment shown in FIG.  2 . 
   Firstly,  FIG. 10  is a block diagram illustrating the constitutions of the child voice gateways  64  and  66 . As shown in this diagram, the child voice gateways  64  and  66  each comprise a central control unit  130 , a memory unit  150 , a voice processing unit  120 , a line switching interface  100 , and a network interface  110 . Herein, the central control unit  130  is connected to the memory unit  150 , the voice processing unit  120 , the line switching interface  100  and the network interface  110 . The line switching interface  100  and the network interface  110  are connected to the voice processing unit  120 . The line switching interface  100  is connected to the telephone switching network  10 . The network interface  110  is connected to the IP network  12 . 
   Secondly, the operations of the child voice gateways  64  and  66  flow as shown in FIG.  12 .  FIG. 12  is a flowchart of the operations of the child voice gateway to transmit the destination inquiring message according to a call-setup request from a telephone switching office. After detecting the call-setup request from the telephone switching office (step S 20 ), the child voice gateway refers to the declaration table thereof and then sends out the destination inquiring message (step S 21 ). After receiving a destination determination message (step S 22 ), the child voice gateway performs the call setup towards the destination (step S 23 ). 
     FIG. 13  is a flowchart of the operations of the destination child voice gateway. After receiving the destination inquiring message (step S 24 ), the destination child voice gateway confirms whether or not the local steering number is identical with the head digits of the destination number (step S 25 ). If it is identical, then the destination child voice gateway transmits a destination determination message to the source child voice gateway (step S 26 ). If not identical, then the destination child voice gateway sends out a failure message (step S 27 ). The above-described operations are performed by an inquiring mechanism  135  and a declaration mechanism  134  of the central control unit  130 . 
   Also, the declaration mechanism  134  of the central control unit  130  shown in  FIG. 10  functions to declare a local transport address  151  or a local steering number  152  to other parent voice gateways (step S 7  of FIG.  5 ). 
     FIG. 14  is a flowchart of the operations of the child voice gateway to transmit the destination inquiring message according to a route selection table  153 . 
   After receiving a start signal (step S 30 ), the child voice gateway refers to the route selection table  153  (step S 31 ) and then confirms whether or not the destination steering number is recorded in the route selection table  153  (step S 32 ). If it is recorded, then the child voice gateway performs a call setup (step S 36 ). If it is not recorded, then the child voice gateway sends out the destination inquiring message (step S 33 ). When receiving a destination determination message (step S 34 ), the child voice gateway records a transport address and steering number of the destination in the route selection table  153  (step S 35 ) and then performs a call setup (step S 36 ). The above-describe operations are performed by the declaration mechanism  134  and the inquiring mechanism  135  of the central control unit  130 . 
     FIG. 15  is a flowchart of operations of the child voice gateway to refer to traffic when recording data in the route selection table  153 . 
   After receiving the start signal (step S 40 ), the child voice gateway refers to the route selection table  153  (step S 41 ) and confirms whether or not the destination steering number is recorded in the route selection table  153  (step S 42 ). If it is recorded, then the child voice gateway performs the call setup (step S 48 ). If it is not recorded, then the child voice gateway sends out the destination inquiring message (step S 43 ). After receiving the destination determination message (step S 44 ), the child voice gateway refers to the traffic (step S 45 ) and then confirms whether or not the traffic is heavy (step S 46 ). If it is heavy, then the child voice gateway records the traffic (step S 47 ) and then performs the call setup (step S 48 ). If it is not heavy, then the child voice gateway performs the call setup (step S 48 ) without recording the traffic. The above-described operations are performed by the declaration mechanism  134 , the inquiring mechanism  135  and a traffic referring mechanism  138  of the central control unit  130 . 
     FIG. 16  is a flowchart illustrating a case where a child voice gateway performs a call setup but fails to set up the call according to the route selection table  153 . 
   Herein, since steps S 50  to S 56  for setting up a call are identical with those described in  FIG. 14 , a description thereof is omitted. 
   After receiving a message after the call setup (step S 57 ) the child voice gateway  64  confirms whether or not the after-call-setup message is a message of failing to set up the call (step S 58 ). If the after-call-setup message is the failure message, then the child voice gateway  64  investigates whether or not the failure is due to a destination not being detected (step S 59 ). If YES, then the child voice gateway  64  sends out the destination inquiring message (step S 53 ); if NO, then the child voice gateway  64  releases the call (step S 61 ). If the after-call-setup message is not the failure message, then the child voice gateway  64  continues to set up the call (step S 60 ). The previously described operations are performed by the declaration mechanism  134  and the inquiring mechanism  135  of the central control unit  130 . 
     FIG. 17  is a flowchart of operations of the child voice gateway to record a detection time in the last access field shown in  FIG. 8  in a case where the child voice gateway deletes the transport address and the corresponding steering number from the route selection table  153  if the transport address is not referred to during a predetermined period. 
   The child voice gateway refers to the route selection table  153  in order to send out the destination inquiring message (step S 70 ), and then confirms whether or not the destination is detected in the route selection table (step S 71 ). If it is detected, then the child voice gateway records the detection date and time in the last access field (step S 72 ). The last access field is periodically monitored by a route-selection-table monitoring mechanism  136  of the central control unit  130 , and is deleted from the route selection table  153  if the monitoring mechanism  136  detects that the last access field is not referred to during the predetermined period. 
     FIG. 18  a flowchart of operations of the child voice gateway in a case of failing to transmit the destination inquiring message to the parent voice gateway. 
   In this diagram after detecting a call-setup request (step S 80 ), the child voice gateway refers to the route selection table  153  (step S 81 ) and then confirms whether or not the destination is detected in the route selection table  153  (step S 82 ). If it is detected, then the child voice gateway performs the call setup (step S 87 ). If it is not detected, then the child voice gateway sends out the destination inquiring message (step S 83 ). After receiving a result of the destination inquiring message (step S 84 ), the child voice gateway investigates the result (step S 85 ). If the result is a destination determination message, then the child voice gateway records the result on the route selection table  153  (step S 86 ); if not, the child voice gateway performs a call setup towards the telephone switching network  10  (step S 88 ). The above-described operations are performed by the inquiring mechanism  135 , the declaration mechanism  134 , and a routing mechanism  139  for the telephone switching network of the central control unit  130 . 
   Next, a description will be given with respect to a case where the local transport address  151  or the local steering number  152  of the child voice gateway is changed. 
   For example, when the local steering number  152  held in the memory unit  150  of the child voice gateway  64  is changed from “722” to “721” by the system operator, the changed local steering number makes an update flag “TRUE” as shown by a table (b) of  FIG. 19. A  local information monitoring mechanism  137  for periodically monitoring the flag detects that the flag becomes “TRUE” and then notifies the declaration mechanism  134  of the detection. The declaration mechanism  134  refers to a declaration table  154  stored in the memory unit  150  (see  FIG. 3 ) and then transmits a declaration message (including the declaration header, the steering number  721 , and the transport address (IP address: 20. 20. 20. 2, port number: 8001)) towards the declaration destination transport address (IP address: 20. 20. 20. 1, port number: 8001). The transmission route is identical with that of S 7  shown in FIG.  5 . 
   Next, a description will be given of constitutions and operations of the parent voice gateways  60  and  62  in the previously described embodiment of FIG.  2 . 
   Firstly,  FIG. 11  is a block diagram illustrating the constitutions of the parent voice gateways  60  and  62 . As shown in this diagram, the parent voice gateways  60  and  62  each comprise a central control unit  230 , a memory unit  250 , a voice processing unit  220 , a line switching interface  200 , and a network interface  210 . Herein, the central control unit  230  is connected to the memory unit  250 , the voice processing unit  220 , the line switching interface  200  and the network interface  210 . The line switching interface  200  and the network interface  210  are connected to the voice processing unit  220 . The network interface  210  is connected to the IP network  12 . 
   Secondly,  FIG. 20  is a flowchart of the parent voice gateway to receive the destination inquiring message from the child voice gateway. When receiving the destination inquiring message from the network interface  210  (step S 100 ), the parent voice gateway refers to the route selection table  253  (step S 101 ) and then sends the destination inquiring message towards a transport address obtained from the route selection table  253  (step S 102 ). These operations are performed by a declaration mechanism  234  and an inquiring mechanism  235  of the central control unit  230 . 
   Also, the declaration mechanism  234  of the central control unit  230  functions to declare a local transport address  251  or a local steering number  252  of the parent voice gateway to the other parent voice gateways as well as to receive declaration messages from the other parent voice gateways or from the other child voice gateways and then record these messages in the route selection table  253 . 
   Returning to the previously described  FIG. 17 , this diagram can also be used to show a flowchart of operations of the parent voice gateway to record a detection date and time on the last access field of  FIG. 8 , with respect to the parent voice gateway deleting the transport address and the corresponding steering number from the route selection table  253  in a case where the transport address is not referred to during the predetermined period. 
   The parent voice gateway refers to the route selection table  253  in order to send out the destination inquiring message (step S 70 ), and then confirms whether or not the destination is detected in the route selection table  253  (step S 71 ). If it is detected, then the parent voice gateway records the detection date and time on the last access field (step S 72 ). The last access field is periodically monitored by a route-selection-table monitoring mechanism  236  of the central control unit  230 , and is deleted from the route selection table  253  if the monitoring mechanism  236  detects that the last access field is not referred to during the predetermined period. 
     FIG. 21  is a flowchart of the parent voice gateway in a case of failing to transmit the destination inquiring message. The detailed example has been described by FIG.  9 . 
   After receiving the destination inquiring message (step S 110 ) the parent voice gateway refers to the route selection table  235  (step S 111 ) and then confirms whether or not the destination is detected in the route selection table  253  (step S 112 ). If the destination is detected, then the parent voice gateway sends out the destination determination message (step S 113 ); if not, the parent voice gateway transmits the failure message towards the child voice gateway (step S 114 ). These operations are performed by the declaration mechanism  234  and inquiring mechanism  235  of the central control unit  230 . 
   Herein, operations in a case where the local transport address  251  or the local steering number  252  of the parent voice gateway is changed are similar to those of the previously described child voice gateway, and a description thereof is omitted. 
   Next, a description will be given with respect to the memory units  150  and  250  for recording control programs of the voice gateways according to the present invention. 
   Firstly, the memory unit  150  as a memory medium functions to store programs for actuating the central control unit  130 . By installing the programs of the present invention into the existing voice gateways, the existing voice gateways can function as those of the present invention. The memory unit  150  as such a memory medium for storing the programs may be, for example, an electronic memory, a hard disk, an optical magnetic memory, a disk tape, or the like. 
   Secondly, the memory unit  250  as the memory medium functions to store programs for actuating the central control unit  230 . By installing the programs of the present invention into the existing voice gateways, the existing voice gateways can function as those of the present invention. The memory unit  250  as the memory medium for storing the programs may be, for example, an electronic memory, a hard disk, an optical magnetic memory, a disk tape, or the like. 
   The above description is provided in order to enable any person skilled in the art to make and use the invention and sets forth the best mode contemplated by the inventions of carrying out their invention. 
   The present invention is not limited to the specifically disclosed embodiment, and variations and modifications may be made without departing from the scope of the present invention. 
   For example,  FIG. 5  shows a case where a signal for a call setup is transmitted from the telephone terminal  74  connected to the telephone switching office  70 . In practice, the call setup can be performed by the previously described method in a case where the call setup signal is transmitted from any telephone terminal connected to any switching office within the telephone switching network  10  or connected to the child voice gateway  64 . Further, the child voice gateway and the parent voice gateway may be combined into one voice gateway, and a plurality of the voice gateways and a plurality of the switching offices may be connected with each other. The IP network  12  may be the Internet or a leased TCP/IP network. The telephone switching network  10  may be a public telephone switching network or a leased network. 
   The present application is based on Japanese priority application No. 10-250092 filed on Sep. 3, 1998, the entire contents of which are hereby incorporated for reference.