Patent Publication Number: US-2007110038-A1

Title: Mobile communication system, call control server, and call control method

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the invention  
      The present invention relates to a mobile communications system, call control server, and call control method and, more particularly, to a mobile communications system which performs communication among a mobile communications system which is a circuit switched network, a mobile communications system which is an IP (Internet Protocol) network, and a PSTN (Public Switched Telephone Network) which is a circuit switched network, the former two of which are run by different telecommunications carriers.  
      2. Description of the related art  
      The tendency to use, as networks constituting ground networks for a mobile communications system, a ground network employing a packet switching system typified by an IP network as well as a conventional network employing a circuit switching system is growing. There are developed technologies which allow communication between mobile terminals in an environment where a circuit switched network and a packet switched network coexist as ground networks for a mobile communications system in whichever mobile terminals are located in the circuit switched network or the packet switched network. For example, in a mobile communications system disclosed in Japanese Patent Laid-Open No. 2002-186011, a mobile communications network employing a circuit switching system and an IP network employing a packet switching system coexist, and a location management computer which manages the location of a mobile terminal is provided for each of the networks. As location management computers, an HLR (Home Location Register) which performs location management in the circuit switched network and a SIP (Session Initiation Protocol) server which performs location management in the IP network are installed.  
      If a mobile terminal is located in the IP network, the mobile terminal makes a location registration request to the IP network. The SIP server registers the location of the mobile terminal and makes a location registration request to the HLR. With this operation, the location of the mobile terminal located in the IP network is also registered by the HLR, which performs location management in the circuit switched network. If communications repeater equipment which relays communication among a PSTN, the IP network, and the circuit switched network receives an incoming request addressed to a mobile terminal from the PSTN, it determines whether the mobile terminal is located in the mobile communications network or in the IP network on the basis of the location information of the mobile terminal registered in the HLR. The communications repeater equipment relays communication with the mobile terminal through the network, which covers the mobile terminal.  
      The network configuration of the mobile communications system disclosed in Japanese Patent Laid-Open No. 2002-186011 is premised on the use of a circuit switched network and a packet switched network provided by a single telecommunications carrier. Accordingly, a mobile terminal need not use different telephone numbers for the circuit switched network and packet switched network, and can accept an incoming call using one telephone number.  
      However, if a mobile telecommunications carrier which uses a packet switched network as a ground network and a mobile telecommunications carrier which uses a circuit switched network as a ground network are different from each other, since the networks are operated under different number systems, it is difficult for a mobile terminal to communicate by using a single telephone number. Therefore, if the mobile telecommunications carriers are different and different telephone numbers are assigned to the mobile terminal for the mobile telecommunications carrier, i.e., the circuit switched network and packet switched network, the technology described in Japanese Patent Laid-Open No. 2002-186011 cannot implement a one-number system in which one of the telephone numbers is used as a universal telephone number of the mobile terminal at which an incoming call is accepted.  
     SUMMARY OF THE INVENTION  
      An exemplary feature of the invention is to provide a mobile communications system, call control server, and call control method capable of implementing a one-number system in which even if a mobile terminal assigned different telephone numbers for respective mobile telecommunications carriers uses different telephone numbers in networks (a circuit switched network and packet switched network), one of the telephone numbers is used as a universal telephone number at which an incoming request is accepted.  
      The mobile communications system according to the present invention has a subscriber database and a call control server. The subscriber database, located in a second mobile communications network, manages subscriber information of a mobile terminal and registers location information in response to a location registration request made by the mobile terminal to the second mobile communications network, and registers location information in response to a location registration request made by the mobile terminal to a first mobile communications network. The call control server changes a caller number in a call request to a telephone number assigned for the second mobile communications network and transmits the call request to a connection destination upon receiving the call request from the mobile terminal which is located under the first mobile communications network. When the location registration request is made by the mobile terminal to the first mobile communications network, the location information is also registered in a Home Location Register in the first mobile communications network. A call control server according to the present invention has a location registration control unit and a call control unit. The location registration control unit requests a subscriber database to register location information in response to a location registration request made by the mobile terminal in the second mobile communications network and obtains subscriber information of the mobile terminal from the subscriber database. The call control unit changes a caller number in a call request to the telephone number assigned for the second mobile communications network and transmits the call request to a connection destination upon receiving the call request from the mobile terminal which is located under the first mobile communications network.  
      A call control method according to the present invention has a first registering step, a second registering step and a call request transmitting step. The first registering step is for registering location information of the mobile terminal to a subscriber database through a call control server which is installed in the second mobile communications network upon receiving a location registration request from the mobile terminal in the second mobile communications network. The second registering step is for registering location information of the mobile terminal to a subscriber database through a mobile switching system which is installed in the first mobile communications network upon receiving a location registration request from the mobile terminal in the first mobile communications network. The call request transmitting step is performed by the call control server to replace a caller number in a call request from the telephone number assigned for the first mobile communications network to the telephone number assigned for the second mobile communications network, and to transmit the call request to a connection destination upon receiving the call request from the mobile terminal located in the first mobile communications network.  
      With the above configuration, according to the present invention, the mobile communications system can implement a one-number system in which even if a mobile terminal assigned different telephone numbers for respective mobile telecommunications carriers uses different telephone numbers in a circuit switched network or packet switched network, one of the telephone numbers is used as a universal telephone number at which an incoming request is accepted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other exemplary features, and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings wherein:  
       FIG. 1  is a block diagram showing the configuration of a mobile communications system according to an exemplary embodiment of the present invention;  
       FIG. 2  is a sequence chart showing the operation for location registration performed by a UE shown in  FIG. 1 ;  
       FIG. 3  is a flow chart showing the control operation performed by a mobile switching system in  FIG. 2 ;  
       FIG. 4  is a sequence chart showing the operation for outgoing connection when the UE shown in  FIG. 1  originates a telephone call;  
       FIG. 5  is a flow chart showing the control operation performed by the mobile switching system in  FIG. 4  when the UE under the mobile communications network originates the telephone call;  
       FIG. 6  is a flow chart showing the control operation performed by a call control server in  FIG. 4  when the UE under the mobile communications network originates the telephone call;  
       FIG. 7  is a sequence chart showing the operation for incoming connection to the UE shown in  FIG. 1 ;  
       FIG. 8  is a flow chart showing the control operation performed by the call control server in  FIG. 7  for the incoming connection to the UE;  
       FIG. 9  is a block diagram showing another configuration of the mobile communications system according to the embodiment of the present invention;  
       FIG. 10  is a flow chart showing the control operation performed by the UE in  FIG. 4  when the UE originates a telephone call;  
       FIG. 11  is a flow chart showing the control operation performed by the call control server in  FIG. 4  when the UE originates the telephone call;  
       FIG. 12  is a flow chart showing the control operation performed by the call control server in  FIG. 7  for the incoming connection to the UE; and  
       FIG. 13  is a flow chart showing the control operation performed by the UE in  FIG. 7  for the incoming connection to the UE; and  
       FIG. 14  is a block diagram showing the call control server according to the embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS  
      Exemplary embodiments of the present invention will be explained next.  
      The present invention relates to call origination and termination operation using a UE (User Equipment) which has double telephone numbers assigned by a telecommunications carrier providing a communications service in an IP network and a telecommunications carrier providing a communications service in a circuit switched network respectively, and characterized by an operation for location registration performed in respective networks, i.e., the IP network and the circuit switched network.  
      The present invention also implements a one-number system. In the one-number system, a call control server having accepted a call addressed to the UE by the telephone number for the IP network performs control to convert the telephone number for the IP network into that for the circuit switched network when transferring the call to the UE located in the circuit switched network, and the call transferred to the circuit switched network is terminated to the UE using the telephone number for the circuit switched network.  
      A detailed explanation will be given below with reference to the drawings.  
       FIG. 1  is a diagram showing the configuration of a mobile communications system according to an exemplary embodiment of the present invention. An IP network  5  and a mobile communications network  15  which is the circuit switched network shown in  FIG. 1  each cover a UE  21  through a radio channel, i.e., the UE  21  can perform communications through either network. A call control server  1 , a media gateway  2 , and a subscriber database  4  belong to the IP network  5 . Each of the call control server  1  and media gateway  2  is connected to a PSTN (Public Switched Telephone Network)  31  and the mobile communications network  15 . The call control server  1  controls transmission and reception of signaling for setting a voice path in the IP network  5 , and it has three main control units as shown in  FIG. 14 . A communication control unit  201  performs communication control for communication with the media gateway  2 , the subscriber database  4 , the PSTN  31 , the IP network  5  and the mobile communications network  15  respectively. A location registration control unit  202  is provided for location registration control to be performed by the present invention which will be described later. A call control unit  203  is provided for call control to be performed by the present invention which will be also described later. The media gateway  2  controls conversion of voice data into media data required in each network at the time of communication among the PSTN  31 , IP network  5 , and mobile communications network  15 . More specifically, the media gateway  2  transmits and receives actual voice data and, for example, converts an IP voice packet into ISDN (Integrated Service Digital Network) voice and vice versa between the IP network  5  and the PSTN  31  or mobile communications network  15 .  
      The interface of the call control server  1  with the IP network  5  or media gateway  2  conforms to IP (Internet Protocol), and the interface thereof with the PSTN  31  conforms to a common channel signaling system. Similarly, the interface of the media gateway  2  with the IP network  5  conforms to IP, and the interface thereof with the PSTN  31  conforms to a common channel signaling system.  
      The subscriber database  4  manages, in a way common to the IP network  5  and mobile communications network  15 , the subscriber information of the UE  21  which has double telephone numbers as one for the IP network  5  and one for the mobile communications network  15 . The interface of the subscriber database  4  with the call control server  1  conforms to IP. The subscriber database  4  performs terminal authentication processing and location information management for the UE  21 .  
      Signaling and an IP voice packet in the IP network  5  are received at an AP (Access Point)  3  and transmitted from the AP  3  to the UE  21  through a radio channel. If the AP  3  uses a wireless LAN as a radio channel, the mechanism of communication between the IP network and the UE  21  becomes the same as that of IP telephone service using a commonly used wireless LAN.  
      A mobile switching system  11 , base station control equipment  12 , a base station  13 , and an HLR  14  belong to the mobile communications network  15 , which is a circuit switched network. In the mobile communications network  15 , voice communication is performed through the mobile switching system  11 , base station control equipment  12 , and base station  13 . The base station  13  and a UE  21  are connected to each other through a radio access channel such as W-CDMA (Wideband-Code Division Multiple Access) or CDMA2000. The HLR  14  manages various types of data on a UE covered by the mobile communications network  15  and performs terminal authentication processing whether the UE can use the mobile communications network  15 , when the UE is to use the mobile communications network  15 . The HLR  14  is connected to the mobile switching system  11  using a protocol conforming to the mobile communications system.  
      The media gateway  2  and mobile switching system  11  communicate a voice packet conforming to ISDN with each other. The call control server  1  and mobile switching system  11  exchange information with each other using signaling conforming to a common channel signaling system. The mobile switching system  11  has interface means for reading out subscriber information from the subscriber database  4  using a protocol conforming to the mobile communications system.  
      In a conventional configuration of the mobile communications network, HLR is provided for a server which manages location information and subscriber information for respective UEs covered by the mobile communications network. However, in this exemplary embodiment, the HLR  14  is used for performing the terminal authentication processing whether the UE  21  can be allowed to access to the mobile communications network  15 , and the actual subscriber information for the UE  21  is stored and managed by the subscriber database  4  in the IP network. Also, in the conventional configuration of the mobile communications system, a mechanism is often used to transfer information in the HLR  14  to a VLR (Visitor Location Register) of a network as a destination to visit. However, in this exemplary embodiment, a VLR is incorporated in the mobile switching system  11 .  
      The operation will be explained next.  
       FIG. 2  is a sequence chart showing the operation for location registration performed by the UE  21  shown in  FIG. 1 . Location registration operation performed by the UE  21  will be explained with reference to  FIG. 2 .  
      When the UE  21  puts power-on or detects a change in location where it is present, it makes a location registration request.  
      If the UE  21  is under the IP network  5 , it knows, from information notified by the AP  3 , that it is located in the IP network  5 , and thus, it uses a telephone number for the IP network  5  as a caller number when it accesses the call control server  1  (step S 1 ).  
      The call control server  1  requests the subscriber database  4  to register the location information of the UE  21  based on the information contained in a location registration request transmitted from the UE  21  (step S 2 ). The subscriber database  4  attempts to perform terminal authentication using the terminal ID of the UE  21 .  
      If the UE  21  is properly authenticated, the subscriber database  4  registers the location information (step S 3 ). Then, the subscriber database  4  transfers the subscriber information of the UE having made the location registration request to the call control server  1  (step S 4 ).  
      The call control server  1  responds to the subscriber database  4  to the effect that it has received the transferred subscriber information (step S 5 ). Upon receipt of the response to the transfer of the subscriber information from the call control server  1 , the subscriber database  4  notifies the call control server  1  of completion of the location registration (step S 6 ). Then, the call control server  1  notifies the UE  21  of completion of the location registration (step S 7 ).  
      On the other hand, if the UE  21  is under the mobile communications network  15 , it accesses the mobile switching system  11  via the base station  13  and base station control equipment  12  in the mobile communications network  15  (step S 11 ). Since the UE  21  knows, from information notified by the base station  13 , that it is located in the mobile communications network  15 , it uses a telephone number for the mobile communications network  15  as a caller number.  
      The mobile switching system  11  first makes a location registration request to the HLR  14 , when it detects that the subscriber information of the UE  21  is managed by the subscriber database  4  in the IP network  5  (step S 12 ). If the HLR  14  identifies that the UE  21  is a valid terminal in the mobile communications network  15 , and the terminal is properly authenticated, the HLR  14  registers the location of the UE  21  (step S 13 ). The HLR  14  registers the location information of the UE  21  and notifies the mobile switching system  11  of completion of terminal authentication and location registration of the UE  21  (step S 14 ).  
      The mobile switching system  11  then makes a location registration request to the subscriber database  4  (step S 15 ). In location registration at this time, only identification information of the mobile switching system  11  which performs the location registration request may be registered as location information in the subscriber database  4  for indicating that the UE  21  is under the mobile communications network  15 . The detailed location information of the UE  21  such as in which location registration area the UE  21  is present need not be registered. This is because information managed by the communications carrier having the mobile communications network should not be registered unnecessarily in the subscriber database  4  which is managed by the different communications carrier.  
      The subscriber database  4  determines whether the UE  21  is a valid terminal for using both networks of the IP network  5  and the mobile communications network  15 . If the determination result is affirmative, the subscriber database  4  registers the location of the UE  21  (step S 16 ) and transfers the subscriber information of the UE  21  to the mobile switching system  11  (step S 17 ). Upon receipt of a response to the transfer of the subscriber information from the mobile switching system  11  (step S 18 ), the subscriber database  4  notifies the mobile switching system  11  of completion of the location registration (step S 19 ). The mobile switching system  11  notifies the UE  21  of the completion of the location registration through the mobile communications network  15  (step S 20 ).  
      The method for the mobile switching system  11  to detect that the subscriber information of the UE  21  is managed by the subscriber database  4  in the IP network  5  can be performed based on information such as subscriber ID or terminal ID which is to be transmitted from the UE  21  included in the location registration request. The mobile switching system  11  judges whether the UE  21  is an accessible terminal not only to the mobile communications network  15  but also to the IP network  5  based on the information of the subscriber ID or the terminal ID. If it detects that the UE  21  is a valid terminal, the mobile switching system  11  judges that the subscriber information of the UE  21  is managed by the subscriber database  4  in the IP network  5 . Note that if the UE  21  is determined as a terminal incapable of connecting to the IP network  5 , the location registration operation becomes the same as that in a conventional mobile communications network, i.e., terminal authentication and location registration of the UE  21  are performed only in the HLR  14 .  
       FIG. 3  is a flow chart showing the control operation performed by a mobile switching system in  FIG. 2  when the UE  21  is under the mobile communications network  15 .  
      When the location registration request is performed by the UE  21  under the mobile communications network  15  (step S 21 ), the mobile switching system  11  judges whether subscriber information of the UE  21  is managed in the IP network  5  or in the mobile communications network  15  (step S 22 ). When the mobile switching system  11  detects that the subscriber information of the UE  21  is managed in the mobile communication network  15 , the mobile switching system  11  transmits the location registration request to the HLR  14  for location registration to be performed by the HLR  14  as an ordinary mobile phone (step S 22 : NO). Then, the mobile switching system  11  obtains the subscriber information of the UE  21  from the HLR  14  and completes the location registration processing (step S 26 ).  
      On the other hand, when the mobile switching system  11  detects that the subscriber information of the UE  21  is managed in the IP network  5 , the mobile switching system  11  requests the HLR  14  to perform an authentication of the UE  21  in the mobile communications network  15  (step S 23 ). If the UE  21  is properly authenticated, the mobile switching system  11  requests the subscriber database  4  in the IP network  5  to perform the location registration by transmitting the location registration request (step S 24 ). The mobile switching system  11 , then, receives the subscriber data of the UE  21  from the subscriber database  4  and completes the location registration processing (step S 25 ).  
      As described above, the location information where the UE  21  is present is registered in the subscriber database  4  in the IP network  5  through the location registration request performed by the UE  21 . Also, as described above, if the UE  21  is under the mobile communications network  15 , only identification information of the mobile switching system  11  which performs the location registration request may be registered as location information in the subscriber database  4  for indicating that the UE  21  is under the mobile communications network  15 .  
      Next, call connection operation of the UE  21  performed in each of the IP network  5  and mobile communications network  15  will be explained.  
      First, outgoing connection from the UE  21  will be explained.  
       FIG. 4  is a sequence chart showing the operation for outgoing connection when the UE shown in  FIG. 1  originates a telephone call.  
      When the UE  21  is under the IP net work  5 , the UE  21  transmits a call request using the telephone number for the IP network  5  as a caller number. The call request is received by the call control server  1  through the AP  3  (step S 31 ). The call control server  1  identifies the connection destination telephone number contained in the call request and determines that the call request is an outgoing connection to the PSTN  31 . The call control server  1  sends out the call request signal to the PSTN  31  (step S 32 ).  
      Note that in communication between the UE  21  and the connection destination, an IP packet transmitted from the UE  21  is relayed by the media gateway  2 , converted into a media signal with a format required for communication with the PSTN  31 , and transmitted. In contrast, a signal from the PSTN  31  is converted into an IP packet at the media gateway  2  and transmitted to the UE  21 .  
      The operation of outgoing connection from the UE  21  which is under the mobile communications network  15  will be explained.  
      The UE  21  transmits a call request using the telephone number for the mobile communications network  15  as a caller number, and the call request is received by the mobile switching system  11  through the base station  13  and base station control equipment  12  within the mobile communications network  15  (step S 33 ).  
      When the mobile switching system  11  identifies, by information contained in the call request, that the subscriber information of the UE  21  is managed in the IP network, the mobile switching system  11  transmits the call request to the call control server  1  in the IP network (step S 34 ).  
      The call control server  1  identifies the connection destination telephone number contained in the call request transmitted from the mobile switching system  11  and determines that the call request is an outgoing connection to the PSTN  31 . The call control server  1  sends out the call request signal to the PSTN  31  (step S 35 ).  
       FIG. 5  is a flow chart showing the control operation performed by the mobile switching system in  FIG. 4  when the UE under the mobile communications network originates the telephone call.  
      When the UE  21  under the mobile communications network  15  originates the telephone call (step S 41 ), the call request for the call is received by the mobile switching system  11 . The mobile switching system  11  judges whether the subscriber information of the UE  21  is managed in the IP network  5  or in the mobile communications network  15  by using information contained in the call request (step S 42 ). If the mobile switching system  11  identifies that the subscriber information of the UE  21  is managed in the mobile communications network  15  (step S 42 : NO), it performs call processing for the call as an ordinary mobile phone (step S 44 ). If the mobile switching system  11  identifies that the subscriber information of the UE  21  is managed in the IP network  5  (step S 42 : YES), the mobile switching system  11  transmits the call request to the call control server  1  in the IP network (step S 43 ).  
      The operation of the call control server  1  at the time of call origination by the UE  21  under the mobile communications network  15  will be explained.  
       FIG. 6  is a flow chart showing the control operation performed by the call control server in  FIG. 4  when the UE under the mobile communications network originates the telephone call.  
      When the UE  21  under the mobile communications network  15  has originated the telephone call, the mobile switching system  11  transmits the call request to the call control server  1  in the IP network at the step S 43  in  FIG. 5  (also, step S 34  in  FIG. 4 ). The call request transmitted from the mobile switching system  11  is received at the call control server  1  in the IP network  5  (step S 51  in  FIG. 6 ).  
      When the call control server  1  receives the call request from the mobile switching system  11 , it identifies the connection destination of the call by using information contained in the call request (step S 52 ). When the call control server  1  identifies that the connection destination of the call is to a terminal under the IP network  5 , which is under controlled by the call control server  1  (step S 52 : YES), it performs ordinary call termination processing in the IP network  5  (step S 53 ). On the other hand, when the call control server  1  identifies that the connection destination of the call is not to a terminal under the IP network  5  but to the PSTN  31  (step S 52 : NO), it transmits the call request to the PSTN  31  as a transferred call (step S 54 ).  
      Note that communication between the UE  21  and the connection destination is relayed by the media gateway  2 .  
      The operation of terminating an incoming call addressed to the UE will be explained.  
       FIG. 7  is a sequence chart showing the operation for incoming connection to the UE shown in  FIG. 1   
      An incoming request from the PSTN  31  is first received by the call control server  1  (step S 61 ). The call control server  1  having received the incoming request inquires of the subscriber database  4  where the UE  21  as a call destination is present (step S 62 ).  
      As explained with reference to the location registration operation, subscriber information including the location information of the UE  21  is managed by the subscriber database  4 . Accordingly, the subscriber database  4  having been inquired of the location information of the UE  21  by the call control server  1  notifies the call control server  1  of the location information on the basis of location information it manages.  
      If the UE  21  is under the IP network  5 , the subscriber database  4  returns the location information of the UE  21  in the IP network  5  registered therein to the call control server  1  (step S 63 ). The call control server  1  having received the location information of the UE  21 , performs incoming call termination processing to the UE  21  via the AP  3  connected to the IP network  5  (step S 64 ).  
      On the other hand, if the UE  21  is present within the mobile communications network  15 , the subscriber database  4  identifies that the UE  21  is under the mobile communications network  15  by detecting the identification information of the mobile switching system  11  which has been registered in the subscriber database  4  as the location information of the UE  21 . Then, the subscriber database  4  sends an inquiry to the mobile switching system  11  for requesting the location information of the UE  21  managed in the mobile communications network  15  (step S 65 ). On the response to the inquiry, the mobile switching system  11  returns the location information of the UE  21  managed in the mobile communications network  15  to the subscriber database  4  (step S 66 ). Then, the subscriber database  4  returns the location information of the UE  21  received from the mobile switching system  11  to the call control server  1  (step S 67 ). As the result, the call control server  1  knows the mobile switching system  11  which controls the UE  21 , and then transfers the incoming request to the mobile switching system  11  (step S 68 ). The mobile switching system  11 , which has received the incoming request from the call control server  1 , performs incoming call termination processing to the UE  21  via the base station control equipment  12  and base station  13  in the mobile communications network  15  (step S 69 ).  
       FIG. 8  is a flow chart showing the control operation performed by the call control server in  FIG. 7  for the incoming connection to the UE.  
      The call control server  1  receives the incoming request for the UE  21  (step S 71 ). The call control server  1  having received the incoming request inquires of the subscriber database  4  where the UE  21  as a call destination is present (step S 72 ). As the result of the inquiry, if it is identified that the UE  21  is located in the IP network  5  (step S 73 : YES), the call control server  1  transmits the incoming request to the UE  21  in the IP network  5  as explained in the step S 64  of  FIG. 7  (step S 74 ). On the other hand, if it is identified that the UE  21  is located in the mobile communications network  15  (step S 73 : NO), the call control server  1  transmits the incoming request for the UE  21  to the mobile switching system  11  as explained in the steps S 65  to S 68  of  FIG. 7  (step S 75 ).  
       FIG. 9  is a diagram showing another configuration of the mobile communications system according to the embodiment of the present invention.  
      The basic function of equipment shown in  FIG. 9  is the same as equipment shown in  FIG. 1 . However, it is different from the configuration shown in  FIG. 1  in that a mobile switching system  11  is directly connected to the PSTN  31  and that the media gateway  2  is not connected to the mobile switching system.  
      For this reason, even with a network configuration in which the IP network  5  and the mobile communications network  15  cannot be directly connected to each other, both networks can be connected via the PSTN  31 .  
      In this configuration as well, the subscriber database  4  is connected to the call control server  1  and the mobile switching system  11 . Accordingly, the location registration operation and basic control operation such as call routing in each of the IP network  5  and mobile communications network  15  are not different from those explained in the configuration of  FIG. 1 .  
      Next, how the one-number system is implemented in the outgoing connection and incoming connection explained in FIGS.  4  to  8  for the UE which has double telephone numbers assigned for the IP network and the circuit switched network respectively will be explained with referring to FIGS.  10  to  13 . In the one-number system, the call control server having accepted a call addressed to the UE by the telephone number for the IP network performs control to convert the telephone number for the IP network into that for the circuit switched network when transferring the call to the UE located in the circuit switched network, and the call transferred to the circuit switched network is terminated to the UE using the telephone number for the circuit switched network.  
       FIG. 10  is a flow chart showing the control operation performed by the UE in  FIG. 4  when the UE originates a telephone call.  
      The UE  21  is aware of the network, the IP network  5  or the mobile communications network  15 , to which it has performed the location registration. Therefore, when it originates a telephone call (step S 81  of  FIG. 10 ), it specifies the telephone number of the UE  21  for the IP network  5  as a caller number for accessing to the IP network  5  (step S 83  of  FIG. 10 , and step S 31  of  FIG. 4 ) if it has performed the location registration in the IP network  5  (step S 82 : YES of  FIG. 10 ). On the other hand, if it has performed the location registration in the mobile communications network  15  (step S 82 : NO of  FIG. 10 ), it specifies the telephone number of the UE  21  for the mobile communications network  15  as a caller number for accessing to the mobile communications network  15  (step S 84  of  FIG. 10 , and step S 31  of  FIG. 4 ).  
      The call request includes the caller number specified by the UE  21  together with the call destination telephone number, and is transmitted to the call control server  1  in the IP network  5  or the mobile switching system  11  in the mobile communications network  15  for an outgoing connection processing.  
       FIG. 11  is a flow chart showing the control operation performed by the call control server  1  in  FIG. 4  when the UE  21  originates the telephone call.  
      When the call control server  1  has received the call request of the UE  21  through the IP network  5  (steps S 91  and S 92 : YES of  FIG. 11 ), the call control server  1  stays the caller number in the call request as it is and transmits the call request to the PSTN  31  with the telephone number for the IP network  5  as the caller number (step S 93  of  FIG. 11  and step S 32  of  FIG. 4 ). On the other hand, when the call control server  1  has received the call request of the UE  21  through the mobile switching system  11  in the mobile communications network  15  (steps S 91  and S 92 : NO of  FIG. 11 , and step S 34  of  FIG. 4 ), the call control server  1  converts the caller number in the call request from the telephone number for the mobile communications network  15  to the telephone number for the IP network  5 , and transmits the call request to the PSTN  31  with the telephone number for the IP network  5  as the caller number (step S 94  of  FIG. 11  and step S 35  of  FIG. 4 ).  
       FIG. 12  is a flow chart showing the control operation performed by the call control server  1  in  FIG. 7  for the incoming connection to the UE  21 .  
      When the call control server  1  has received the incoming request to the UE  21  (step S 101  of  FIG. 12  and step S 61  of  FIG. 7 ), the call control server  1  judges whether the UE  21  is located in the IP network  5  or in the mobile communications network  15  by inquiring to the subscriber database  4  (step S 102  of  FIG. 12  and step S 62  of  FIG. 7 ).  
      If the call control server  1  has detected that the UE  21  is under the IP network  5 , the call control server  1  performs the call termination processing to the UE  21  without converting the terminating telephone number which is specified as the telephone number for the IP network  5  in the incoming request (steps S 102 : YES and S 103  of  FIG. 12 . and step S 64  of  FIG. 7 ). On the other hand, if the call control server  1  has detected that the UE  21  is under the mobile communications network  15  (step S 102 : NO of  FIG. 12 ), the call control server  1  converts the terminating telephone number in the incoming request from the telephone number for the IP network  5  to the telephone number for the mobile communications network  15 , and transfers the incoming request to the mobile switching system  11  in the mobile communications network  15  (step S 104  of  FIG. 12  and step S 68  of  FIG. 7 ). The mobile switching system  11  performs the call termination processing to the UE  21  using the telephone number for the mobile communications network  15  in the incoming request (step S 69  of  FIG. 7 ).  
       FIG. 13  is a flow chart showing the control operation performed by the UE  21  in  FIG. 7  for the incoming connection to the UE  21 .  
      When the incoming connection is terminated to the UE  21  (step S 111 ), the UE  21  stays the terminating telephone number as it is which is the telephone number for the IP network  5 , if the incoming connection has been processed in the IP network  5  (steps S 112 : YES and S 113  of  FIG. 13 ). On the other hand, the UE  21  converts the terminating telephone number from the telephone number for the mobile communications network  15  to the telephone number for the IP network  5  if the incoming connection has been processed in the mobile communications network  15  (steps S 112 : NO and S 114  of  FIG. 13 ).  
      As has been explained above, according to the present invention, the one-number system can be implemented by the call control server  1  and the UE  21  which perform control operations shown in FIGS.  10  to  13 . Especially, the call control server  1  converts a caller number to a telephone number for the IP network  5  in a call request transferred from the mobile switching system  11  when it transmits the call request to the PSTN  31 , and also converts a terminating telephone number to a telephone number for the mobile communications network  15  in an incoming request received from the PSTN  31  when it transfers the incoming request to the mobile switching system  11 . By the processing performed in the call control server  1 , the one-number system is realized for the UE  21  which has double telephone numbers, one for the IP network  5  and another for mobile communications network  15 , assigned by respective different telecommunications carriers, and the telephone number for the mobile communications network  15  is used in the mobile communications network  15  although an incoming call is accepted at the call control server  1  by the terminating telephone number which is the telephone number for the IP network  5 .  
      The control operations shown in  FIG. 3 ,  FIG. 5 ,  FIG. 6 ,  FIG. 8  and FIGS.  10  to  13  can also be realized by a computer readable program stored in advance in a storage media such as a ROM (Read Only Memory) which a CPU (Central Processing Unit) reads and executes the processing.  
      While this invention has been described in connection with certain exemplary embodiments, it is to be understood that the subject matter encompassed by way of this invention is not to be limited to those specific embodiment. On the contrary, it is intended for the subject matter of the invention to include all alternative, update, and equivalents as can be included within the spirit and scope of the following claims.  
      Further, the inventers&#39; invention is to retain all equivalents of the claimed invention even if the claims are amended during prosecution.