Abstract:
A method and system for directed call establishment to anchor selected cellular calls in a Voice over Internet Protocol (VoIP) or an IP Multi-Media Subsystem (IMS) network employs a mobile handset application client to interrupt selected cellular call initiations and request a directed call establishment number from a converged services node. A call placed to the directed call establishment number creates a call signaling path through the converged services node in the VoIP or IMS network, which can then exercise control over the call to provide enhanced call services. The converged services node completes a call connection to the original called number in a manner transparent to the user of the mobile handset.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This first filed application for this invention. 
       MICROFICHE APPENDIX 
       [0002]    Not Applicable. 
       TECHNICAL FIELD 
       [0003]    This application relates in general to the delivery of enhanced communications services and, in particular, to a method and system for directed call establishment to facilitate the provision of the enhanced communications services to users of cellular radio devices. 
       BACKGROUND OF THE INVENTION 
       [0004]    The provision and control of communication services in mobile networks such as the Public Land Mobile Network (PLMN) are well understood. However, the services that can be provided within the PLMN are restricted to those supported by the Signaling System 7 (SS7) Advanced Intelligent Network (AIN). More importantly, the features supported within any service providers PLMN are restricted to the SS7/AIN feature set licensed by the network provider, which may not be a full implementation of SS7/AIN. 
         [0005]    With the advent of deregulation and the introduction of Mobile Virtual Network Operators (MVNO), the rapid and cost-effective introduction of enhanced communications services is desirable. Enhanced communications services are most readily developed and deployed within the flexible environment provided in a hosted Voice over Internet Protocol (VoIP) or an IP Multi-Media Subsystem (IMS) network. Since, inter-working between the PSTN/PLMN and the VoIP/IMS networks is now well supported; services developed in a VoIP/IMS network can now be deployed for use by devices operating in the PLMN. 
         [0006]    However, providing those enhanced communications services to PLMN subscribers requires routing control for routing calls placed by enhanced service subscribers to the VoIP/IMS network that supports those services. This poses a problem for MNVOs, as well as for any mobile service provider that supports roaming subscribers. As is well understood by those skilled in the art, normal call routing in the PSTN/PLMN is governed by rigid rules and the dialing plan provisioned within each service provider network. Routing calls along any path other than that dictated by normal call routing in the PLMN requires network provisioning that is frequently impractical or prohibitively expensive to achieve. 
         [0007]    There therefore exists a need for a method and system that permits calls to be established in a directed manner that permits enhanced service provision without originating PLMN network provisioning. 
         [0008]    Furthermore, it is generally more economical for the service provider and the service subscriber to carry roaming subscriber originated calls over a packet data service to a greatest possible extent. It is therefore in the interest of both parties to pass calls to a most economical gateway in order to reduce call carriage expense. 
         [0009]    There therefore exists a need for a method and system that permits calls to be established in the directed manner that reduces the cost of roaming subscriber originated cellular calls. 
       SUMMARY OF THE INVENTION 
       [0010]    It is therefore an object of the invention to provide a method and a system for directed call establishment to reduce the cost of roaming subscriber originated cellular calls and to facilitate the provision of enhanced communications services for any cellular call launched by an enhanced communications service subscriber, regardless of whether the subscriber is roaming. 
         [0011]    The invention therefore provides a method for directed call establishment to anchor selected cellular calls in a Voice over Internet Protocol (VoIP) or an IP Multi-Media Subsystem (IMS) network, comprising: intercepting all cellular calls launched using the mobile handset; sending a DCE request message including the called number to request a DCE dial number (DCE/DN) from a converged services node (CSN); receiving the DCE/DN from the CSN in response to the DCE request message; automatically launching a cellular call from the mobile handset to the DCE/DN; and completing a call to the called number after the CSN receives a call setup request message containing the DCE/DN. 
         [0012]    The invention further provides a system for directed call establishment of selected cellular calls initiated by a subscriber from a mobile handset, comprising: a mobile handset application client programmed to: monitor user input to a mobile handset to intercept selected cellular telephone calls launched using the mobile handset; send a called number associated with each selected cellular call to a converged services node (CSN) to request a directed call establishment (DCE) number; receive a DCE dial number (DCE/DN) from the CSN; and launch a cellular telephone call from the mobile cellular handset using the DCE/DN; and a converged services node, comprising a DCE application programmed to: receive DCE request messages from the mobile handset application client; select a DCE/DN for each DCE request message received; and, return to the mobile handset client application a DCE response message containing the DCE/DN. 
         [0013]    The invention further provides a mobile handset application client, comprising: program instructions for monitoring user input to a mobile handset to intercept selected cellular telephone calls launched using the mobile handset; program instructions for sending a called number associated with the selected cellular calls to a converged services node (CSN) to request a directed call establishment dial number (DCE/DN); program instructions for receiving the DCE/DN from the CSN; and program instructions for launching a cellular telephone call from the mobile cellular handset using the DCE/DN. 
         [0014]    The invention yet further provides a converged services node, comprising: a directed call establishment (DCE) application that receives a DCE request message from a mobile handset client application; stores a called number sent in the DCE request message; selects a DCE dial number (DCE/DN) for the DCE request message; returns a DCE response message containing the DCE/DN to the mobile handset client application; and, completes a call to the called number when a call setup request containing the DCE/DN is received. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
           [0016]      FIG. 1  is a schematic diagram of a hosted VoIP network provisioned with a converged services node (CSN) configured to provide directed call establishment in accordance with the invention; 
           [0017]      FIG. 2  is a schematic diagram of an IP Multi-Media Subsystem (IMS) network provisioned with a CSN configured to provide directed call establishment in accordance with the invention; 
           [0018]      FIG. 3  is a schematic diagram of one embodiment of the CSN provisioned to provide directed call establishment in accordance with the invention; 
           [0019]      FIG. 4  is a block diagram of a single or dual-mode handset provisioned for directed call establishment in accordance with the invention; 
           [0020]      FIG. 5  is a flow diagram providing an overview of tasks performed by the single or dual-mode mobile handset shown in  FIG. 4  while providing a directed call establishment service in accordance with the invention; 
           [0021]      FIG. 6  is a flow diagram providing an overview of tasks performed by the CSN during a first phase of directed call establishment in accordance with the invention; 
           [0022]      FIG. 7  is a flow diagram providing an overview of tasks performed by the CSN during a second phase of directed call establishment in accordance with the invention; and 
           [0023]      FIG. 8  is a message flow diagram schematically illustrating principle messages exchanged between components of the networks shown in  FIGS. 1  or  2  in providing a directed call establishment service in accordance with the invention. 
       
    
    
       [0024]    It should be noted that throughout the appended drawings, like features are identified by like reference numerals. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0025]    The invention provides a system and method for directed call establishment to reduce cost and enable the provision of enhanced services to subscribers originating cellular calls in a Public Land Mobile Network (PLMN). the system includes a mobile handset provisioned with an application client adapted to perform directed call establishment. The mobile handset cooperates with but operates independently of a converged services node (CSN). The CSN may be embodied as a Session Initiation Protocol (SIP) application server in a packet data network. 
         [0026]      FIG. 1  is a schematic diagram of a hosted VoIP network  10  provisioned with a CSN configured to perform directed call establishment in accordance with the invention. As is well understood by those skilled in the art, hosted VoIP networks are connected to untrusted VoIP networks  12  that serve Enterprise and/or home environments. The hosted VoIP network  10  is also connected to the PSTN/PLMN  14  to permit the offering of transparent communications services originated or terminated in any one of networks  12  and  14 . The untrusted VoIP networks  12  are connected to the hosted VoIP network  10  by session border controllers  16 , well known in the art. The PSTN/PLMN network  14  is connected to the hosted VoIP network  10  by Media Gateways  18  and soft switches  34 . 
         [0027]    The hosted VoIP network  10  is provisioned with the CSN  20 , which acts as a SIP Application Server to provide inter-working functions for specific services between the PSTN/PLMN  14  and the VoIP networks  10 ,  12 . 
         [0028]    The hosted VoIP network  10  further includes one or more feature servers  24  which receive incoming communications session requests from the session border controller(s)  16  via communications link(s)  36  in a manner well known in the art. The hosted VoIP network  10  further includes other SIP application servers  26  and media servers  28 , both of which are known in the art. Each of the servers are connected to a core SIP Proxy  30   a  which has global knowledge of the hosted VoIP network  10  and controls intra-network routing. An inter-network routing server  32  provides routing control when calls must be routed to other connected networks  12 ,  14 . Soft switches  34  perform soft switching services within the hosted VoIP network  10 . The soft switches  34  are connected by signaling links  52  to PSTN/PLMN network  14  and are IP connected as indicated at  50  to the Media Gateways  18 . Communication channel  58  connects the session border controllers  16  and the Media Gateways  18 . Trunks  56  connect the Media Gateways  18  to the PSTN/PLMN  14 . IP interfaces  38 ,  40 ,  42 ,  44 ,  46  and  48  respectively connect the feature servers  24 , CSN  20 , SIP application servers  26 , media servers  28 , inter-network routing server  32  and soft switches  34  to the core SIP Proxy  30   a  in a manner well known in the art. IP interfaces  36  and  37  connect the session border controllers  16  to the feature servers  24  and the core SIP Proxy  30   a , likewise in a manner known in the art. 
         [0029]    It should also be noted that the CSN  20  may be connected to the signaling network of the PSTN/PLMN  14  by any version or variant of Transaction Capabilities Application Part (TCAP) signaling links  22 . This permits the CSN  20  to coordinate and control calls originating in the PSTN/PLMN  14 , the hosted VoIP network  10 , or other untrusted VoIP networks  12 , provided that signaling routes provisioned in the respective networks are configured to route signaling messages to the CSN  20  as explained in detail in applicant&#39;s co-pending United States Patent Application Publication No. 20060142010 entitled Method, System and Apparatus for Call Path Reconfiguration filed Dec. 27, 2004, the specification of which is incorporated herein by reference. 
         [0030]      FIG. 2  is a schematic diagram of an IMS network  60  provisioned with the CSN  20 . The IMS network  60  is connected by links  54  to: other untrusted VoIP networks  12  by border control functions  17 ; the PSTN/PLMN  14  by Media Gateways  18 ; and, other IMS domains  62  by signaling links  72  and  74 . In addition to the components described above with reference to  FIG. 3 , the IMS  60  includes a session charging function  66  connected to the CSN  20  by signaling link  84  and a home subscriber server (HSS)  68  connected to the CSN  20  by signaling link  80  and to a proxy/service/interrogating call session control function (P-CSF)  64  by a signaling link  78 . 
         [0031]    A Serving Call Session Control Function (S-CSCF)  30   b  functions in a way similar to the core SIP Proxy  30  described with reference to  FIG. 2 , and is connected to the other network components in the same way. The S-CSCF  30   b  and the P-CSCF  64  are connected to the border control function(s)  17  by signaling links  76 . The S-CSCF  30   b  is also connected to an Interrogating Call Session Control Function (I-CSCF)  65  by a signaling link  70 , which is in turn connected to the Media Gateway control function (MGCF)  18  by a signaling link  71  and to the P-CSCF  64  by a signaling link  82 . The S-CSCF  30   b  is connected to the other IMS domain  62  by a signaling link  74 . The P-CSCF  64  is connected to the other IMS domains by a signaling link  72 . All components, interconnections and operations of all elements of the IMS  60  are well known in the art, with the exception of the CSN  20 . 
         [0032]    As described above with reference to  FIG. 1 , the CSN  20  may be connected to the signaling network of the PSTN/PLMN  14  by any version or variant of Transaction Capabilities Application Part (TCAP) signaling links  22 . This permits the CSN  20  to coordinate and control calls originating in the PSTN/PLMN  14 , the IMS  60 , other IMS domain  62  or untrusted VoIP networks  12 , provided that signaling routes provisioned in the respective networks are configured to route signaling messages to the CSN  20 . 
         [0033]      FIG. 3  is a schematic diagram of one embodiment of the CSN provisioned to provide directed call establishment in accordance with the invention. As explained above, in this embodiment the CSN  20  is a SIP Application Server. The CSN  20  is provisioned with a directed call establishment (DCE) application  88  programmed to function as described below with reference to  FIGS. 6-8 . The CSN  20  is also provisioned with a SIP signaling interface  90 , a data messaging interface  92 , and optionally a SS7 signaling interface  96 . The CSN  20  is also provisioned with a database  98  that is populated with at least one directed call establishment dial number (DCE/DN) pool. As will be explained below in detail, each DCE/DN pool contains dial numbers used to route cellular calls from a single/dual-mode mobile handset  100  ( FIG. 4 ) through a most appropriate gateway to the CSN  20 . A most appropriate gateway may be the most economical to limit cost, or a gateway that supports the required feature set, or any combination of requirements. The number of DCE/DN pools populated in the database  98  is a matter of design choice, Service Level Agreements and other factors understood by those skilled in the art. 
         [0034]      FIG. 4  is a block diagram of a single or dual-mode mobile handset  100  provisioned with a mobile handset application client  102  that is programmed with directed call establishment logic to perform client functions for directed call establishment in accordance with the invention. The application client  102  operates cooperatively with but independently of the CSN  20  to enable the cost savings and enhanced communications services afforded by directed call establishment. 
         [0035]    The application client  102  includes a user interface  104  provisioned with a user interface manager  110 . The user interface manager  110  controls a microphone  112 , a speaker  114 , and a visual display  116  and accepts inputs from a keypad  118  in a manner well known in the art. The application client  102  further optionally includes a call setup and handoff control  106 , which is provisioned with a first line  120  (Line  1 ) and a second line  122  (Line  2 ). Line  1  ( 120 ) and Line  2  ( 122 ) are used to enable subscriber features such as “call waiting”, “3-way conference” and “call hold”, all of which are known in the art. 
         [0036]    Network interfaces  108  support a cellular stack  124 , and if the mobile handset  100  is a dual-mode handset also support a packet network stack  126 . The cellular stack  124  includes a set of layered protocols that are used in existing cellular networks. These protocols are used to send information to and receive information from an MSC via a base station using a cellular radio  128 . Similarly, the packet network stack  126  includes a set of layered protocols for sending and receiving information via a packet network using a packet radio  130 . The application client  102  is either provisioned with an excluded number list  140  or with a query mechanism  141  that permits the application client  102  to query a network database  143  which stores the excluded number list, as will be explained below in more detail with reference to  FIG. 5 . The excluded number list is used to store dial numbers to which directed call establishment is not applied, e.g. emergency numbers and the like. In one embodiment, the excluded number list is pre-provisioned with default excluded numbers, and may be edited by the user to add excluded numbers as desired, or to modify or delete excluded numbers that the user has added. Any call launched to a called number that is not in the excluded number list  140 ,  143  is a “selected call” to which direct call establishment is applied. Although the client shown in this embodiment is for a dual mode mobile handset, directed call establishment can be incorporated into a single mode mobile handset that is not part of a seamless handoff service offering. 
         [0037]      FIG. 5  is a flow diagram providing an overview of tasks performed by the application client  102  of the single or dual-mode mobile handset  100  shown in  FIG. 3 , while providing a directed call establishment service in accordance with the invention. 
         [0038]    The application client  102  of the mobile handset  100  monitors user input ( 202 ) by monitoring the user interface  104  ( FIG. 4 ) to determine when the user launches a new call using the keypad  118  or the speaker  114 , in a manner well known in the art. If the application client  102  detects initiation of a new call ( 204 ), and the mobile handset  100  is a dual-mode device, the application client  102  determines whether the mobile handset  100  is operating in cellular mode ( 206 ). If a dual-mode mobile handset  100  is not operating in cellular mode, the application client  102  processes the new call ( 207 ) using the packet radio  130  and returns to monitoring user input ( 202 ). If the handset  100  is a single mode cellular device, steps  206  and  207  are not performed, as will be understood by those skilled in the art. 
         [0039]    If the mobile handset  100  is operating in cellular mode, the application client  102  determines whether the new call is associated with an excluded number ( 208 ) by referring to the excluded number list  140  or querying the database to refer to the excluded number list  143  ( FIG. 4 ). As noted above, any number may be designated an excluded number; however, the numbers most likely to be placed in the excluded number list are, for example, “911” and the access number for the mobile handset  100  user&#39;s voice mailbox. If the new call has been placed to an excluded number, the application client  102  processes the call in the usual way using the cellular radio ( 210 ). 
         [0040]    If the new call has not been placed to an excluded number, the call is selected for directed call establishment and the application client  102  collects location information ( 212 ). The location information may be collected on a continuous or peroiodic basis, for example on startup; when a handoff from one base station to another is performed; or on a predetermined schedule. The location information can be collected in a number of different ways. Location information (e.g., country code and area code) is routinely provided to the mobile handset  100  by cellular service providers in a manner well known in the art. Alternatively, location information can be derived from a Global Positioning System (GPS) if the mobile handset  100  is equipped with a GPS receiver. The manner in which location information is collected is not important. The location information is useful, however, in enabling a most appropriate packet data network gateway to be selected for a roaming mobile handset  100 , as will be explained below in more detail with reference to  FIG. 6 . 
         [0041]    the application client  102  then composes a directed call establishment (DCE) request message using: current mobile handset location information; mobile handset identification; and, the called number associated with the new call ( 214 ). The handset identification may be, for example, the mobile number associated with the mobile handset  100  or a single directory number if the user subscribes to a single directory number service. Depending on the DCE/DN selection algorithm(s) in the CSN  20 , other information may also be sent in the DCE request message, such as: handset hardware configuration; attached network capabilities; etc. 
         [0042]    The application client  102  then sends the DCE request message to the CSN  20  ( 216 ) using the data messaging channel available through the cellular radio  128 , and waits for a DCE response message. As understood by those skilled in the art, the data channel may be a circuit mode data channel (for example, USSD); a packet mode cellular data channel; a Wide Local Area Network (WLAN) data channel; a Short Message Service (SMS) data channel; a Multimedia Message service (MMS) data channel; or the like. In reply to the data message sent at  212 , the mobile handset  100  receives ( 218 ) a DCE response message containing a directed call establishment dial number (DCE/DN) from the CSN  20  via the data messaging channel. As will be explained below in ore detail with reference to  FIGS. 6-8 , the DCE/DN is a temporary number used to route a signaling path for the call though the CSN  20 . The application client  102  extracts the DCE/DN from the DCE response message and launches a cellular voice call using the DCE/DN as the called number ( 220 ). This establishes a call signaling path to the CSN  20  and anchors the call in a VoIP or an IMS network that hosts the CSN  20 , as will be explained in more detail with reference to  FIGS. 6-8 . 
         [0043]      FIG. 6  is a flow diagram providing an overview of tasks performed by the DCE application  88  operating on the CSN  20  during a first phase of directed call establishment. During the first phase, the CSN  20  monitors its data channel ( 300 ) for receipt of a data message. Each time a data message is received, the CSN  20  determines whether the data message is a DCE request message ( 302 ). If the CSN  20  determines that the data message is to a DCE request message, the CSN  20  performs any processing required by the data message ( 304 ) and returns to monitoring the data channel ( 300 ). If it is determined at  302  that the data message is a DCE request message, the CSN  20  passes the DCE request message to the DCE application  88 , and the DCE application  88  extracts location information, mobile handset  100  identification and the called number from a the DCE request message ( 306 ). 
         [0044]    The DCE application  88  then selects ( 308 ) a DCE/DN from the database  98  ( FIG. 3 ). In one embodiment, the DCE application  88  uses the location information and other optimization logic that exists in the CSN  20  to select a DCE/DN from one of a plurality of DCE/DN number pools indexed such that the location information can be used to locate a specific DCE/DN number pool from which the DCE/DN is selected. The purpose of the location-indexed DCE/DN number pools is to route the call to a most appropriate gateway to a packet data service where the call is anchored to the CSN  20 , which can exercise call control. Each DCE/DN in a DCE/DN number pool is unique to that number pool and, once assigned to a call, the DCE/DN cannot be re-used for another call that requires a DCE/DN from the same number pool until the directed call setup is completed, as will be explained below in more detail. Once the DCE DN is assigned a timer is started. 
         [0045]    Once the DCE/DN is selected and the timer is started at  308 , the DCE application  88  stores a copy of the DCE/DN along with the handset identification, the called number and any other information received in the DCE request message in a memory for later retrieval ( 310 ), as will be further explained below with reference to  FIGS. 7 and 8 . The DCE application  88  then prepares a DCE response to the DCE request message received at  302  to sends the DCE response message ( 312 ) to the mobile handset  100 . 
         [0046]      FIG. 7  is a flow diagram providing an overview of tasks performed by the DCE application  88  during a second phase of directed call establishment. As will be understood by those skilled in the art, the CSN  20  continuously monitors the timer ( 340 ) set at  308 , as well as its call signaling interface(s) for inbound call control messages. If the timer set at  308  expires, it is assumed that the mobile handset  100  is unable to complete the call, and the call session is canceled ( 342 ). The CSN  20  then returns the DCE/DN to the number pool from which it was extracted ( 344 ), and processing of the call ends. 
         [0047]    Each time an inbound call setup request is received, the called number is extracted ( 350 ). The called number is examined to determine whether it is a DCE/DN ( 352 ). If the called number is not a DCE/DN, the CSN  20  performs any required call processing ( 354 ) and returns to monitoring the timer set at  308  and its call signaling interface(s) for inbound call control messages. If the called number is a DCE/DN, the CSN  20  passes the call control message to the DCE application  88 , which uses the DCE/DN (and location information if location information was used at  308  to select the DCE/DN number pool) to retrieve in the called number ( 356 ). The DCE application  88  then returns the DCE/DN to the number pool from which it was extracted ( 358 ) for use by other directed call establishment calls. 
         [0048]    The DCE application  88  then prepares a call setup request ( 360 ) using the handset identification and the called number stored at  310 . As will be understood by those skilled in the art, the handset identification is inserted in the calling party number and the called number is inserted in the called party number of the call setup request. After the call setup request is prepared, the DCE application  88  passes it to the DSN  20 , which forwards the call setup request into the packet network using routing criteria well understood in the art ( 362 ) and directed call establishment ends. Because the CSN  20  is now a signaling node in the call path, it can exercise control over directed call establishment calls to effect special call features when directed to do so using any one of a plurality of special call feature activation procedures. 
         [0049]      FIG. 8  is a message flow diagram schematically illustrating principle messages exchanged between components of the networks shown in  FIGS. 1  or  2  in an example of providing the directed call establishment service. In this example, a B-Party using the mobile handset  100  dials an A-Party&#39;s number ( 400 ). A-Party in this example is using a PSTN telephone  88 , but could be using a fixed-line telephone connected to a PSTN or VoIP network, or a mobile device connected to any one of a PLMN, Wi-FI, Wi-Max or VoIP network. 
         [0050]    When B-Party launches a new call using the mobile handset  100 , the application client  102  intercepts the call launch ( 402 ) and determines whether the number is on the exclusion list, as explained above with reference to  FIG. 5 . The application client  102  then prepares a DCE request message and sends it to the CSN  20  ( 404 ) using data messaging techniques well known in the art. The DCE request message includes location information, A-Party&#39;s number and the handset identification, as also described above with reference to  FIG. 5 . On receipt of the DCE request message, the DCE application  88  extracts the information sent in the DCE request message, and uses the location information by applying an algorithm to select a DCE/DN from a DCE/DN number pool ( 406 ). The DCE application  88  then stores the handset identification and the called number with the DCE/DN ( 408 ). As soon as that information is stored, the DCE application  88  formulates a DCE response message and passes it to the CSN  20 , which sends the DCE response message to the mobile handset  100  ( 410 ). The DCE response message contains the DCE/DN. 
         [0051]    When the mobile handset  100  receives the DCE response message, the application client  102  launches a call to the DCE/DN ( 412 ) provided by the CSN  20  in the DCE response message sent at  410 . As will be appreciated by those skilled in the art, the user of the mobile handset  100  assumes that the call launched at  412  is the call placed to A-Party, because the process described above is entirely transparent to the user of the mobile handset  100 . When the call is launched, the DCE/DN is sent over the PSTN signaling channel ( 414 ) to a Mobile Switching Center (MSC)  92  that is currently serving the mobile handset  100 . The MSC  92  translates the DCE/DN using translation tables well known in the art and determines that an Integrated Services User Digital Part (ISUP) Initial Address Message (IAM) should be sent over a trunk that will direct the message to a Media Gateweay  96  associated with the DCE/DN. The MSC  92  therefore formulates the IAM and forwards it towards the Media Gateweay  96  ( 416 ). On receipt of the IAM, the Media Gateway  96  translates the DCE/DN and determines that it points to the CSN  20 . The Media Gateway  96  therefore formulates a SIP Invite Message and sends it to the CSN  20  ( 418 ). The SIP Invite message contains a called number equal to the DCE/DN; and indication that the call originated from B-Party; and a RTP port number to be used for the B-Party connection. On receipt of the SIP Invite, the CSN  20  returns a SIP  100  Trying message ( 420 ) to the Media Gateway  96 . 
         [0052]    Meanwhile, the CSN  20  passes the DCE/DN to the DCE application  88  which performs DCE/DN correlation ( 424 ) with DCE/DN records stored at  408  to retrieve the information stored at that time. After finding a matching DCE/DN and retrieving the stored information, the DCE application  88  passes it to the CSN  20 , which formulates a SIP Invite message that it sends to the Media Gateway  96  ( 426 ). The SIP Invite message contains a called number equal to the A-Party number; an indication that the call is from the mobile handset  100 ; and, the B-Party RTP Port number provided in the SIP Invite at  418 . The Media Gateway  96  returns a SIP  100  Trying message ( 428 ). The Media Gateway  96  then responds by formulating an IAM message, which it sends into the PSTN signaling network ( 430 ). the IAM includes a calling party number equal to the handset identification, and a called party number equal to the called number, i.e., the A-Party number. 
         [0053]    The called party number causes the IAM to be routed to the PSTN End Office  98 , which serves A-Party telephone  88 . The PSTN End Office  98  verifies that the telephone  88  is on-hook and responds with an Address Complete message (ACM) ( 431 ). The PSTN End Office  98  then applies ringing ( 434 ) to the line that supports the telephone  8 . On receipt of the ACM at  431 , the Media Gateweay  96  sends a SIP  180  Ringing message ( 433 ) to the CSN  20 , which forwards returns a SIP  180  Ringing message towards the B-Party ( 434 ). The Media Gateway  96  responds by returning an Address Complete message (ACM) to the MSC  92  ( 435 ), and ringing applied by the PSTN End Office  90  ( 436 ) is heard by B-Party. 
         [0054]    Meanwhile, on hearing the ringing applied at  432 , A-Party takes the telephone  88  off-hook ( 437 ) and an off-hook signal is returned to the PSTN End Office  98  ( 438 ). On receipt to the off-hook signal, in the PSTN End Office  98  formulates an Answer message (ANM) and sends it to the Media Gateway  96  ( 440 ). On receipt of the ANM, the Media Gateway formulates a SIP  200  OK message and sends that to the CSN  20  ( 442 ). The CSN  20  then returns a SIP  200  OK message ( 444 ) cooresponding to the SIP Invite received at  418 . The Media Gateway responds by sending an ANM to the MSC  92  ( 446 ). Thereafter B-Party is connected to A-Party via the Media Gateway  96  ( 448 ), and the call signaling path extends through the CSN  20 . This anchors the call at the CSN  20  and permits the CSN  20  to exercise control over the call if and when required or requested to do so. 
         [0055]    The invention thereby provides a simple, reliable and economincal mechanism for permitting cellular service providers to support enhanced call services for their roaming subscribers without the use of Service Level Agreements, which entail complex negotiations, costly network provisioning, and the like. 
         [0056]    The invention also reduces costs for service providers and service subscribers by routing cellular calls through a most appropriate gateway to a packet data network where the call is anchored in a converged services node, which can then exercise control over the call in order to provide the enhanced call services. 
         [0057]    It should be understood that the above-described networks, equipment and algorithms are exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.