Conversational bearer negotiation

1. A method of setting up a session between first and second peer user terminals of a communication system, wherein at least one of the peer user terminals is a dual-mode mobile terminal having access to an Internet Protocol (IP) based packet switched network and a circuit switched access network, said session extending at least in part across the circuit switched access network, the method comprising the steps of:

transporting signaling to initiate the session via the IP based packet switched network using a call control protocol which is also used for setting up end-to-end packet switched sessions, the signaling being Session Initiation Protocol (SIP) signaling exchanged between the dual mode mobile terminal and a SIP server of an IP Multimedia Core Network Subsystem (IMS);

sending a notification from the SIP server to a gateway server that one or more conversational bearers must be established, the gateway server terminating the circuit switched session within the system;

after notification from the SIP server, sending from the gateway server to the dual-mode mobile terminal, a callback telephone number; and

calling the callback telephone number by the dual-mode mobile terminal to establish a circuit switched call with the gateway server.

2. The method according to claim 1 , further comprising utilizing the circuit switched call to provide the one or more conversational bearers.

3. The method according to claim 2 , further comprising utilizing the packet switched session to provide non-conversational bearers established over said IP based packet switched network.

4. The method according to claim 1 , wherein said SIP server and said gateway server are co-located.

5. The method according to claim 1 , wherein the gateway server provides interworking between the circuit switched call and the packet switched session.

6. The method according to claim 1 , wherein at least one peer user terminal is notified of the callback telephone number via the SIP server.

7. The method according to claim 6 , wherein the gateway server maps the established circuit switched call to the packet switched session based on the callback telephone number.

8. The method according to claim 1 , wherein the gateway server selects the callback telephone number from a pool of available callback numbers.

9. The method according to claim 1 , further comprising determining by the SIP server that the packet switched session requires setting up a circuit switched call as a result of one or more of the following:

properties of the system known to the SIP server;

prior notification by at least one of the peer user terminals;

information contained in the SIP signaling initiating the packet switched session;

properties defined for at least one of the peer user terminals;

prior notification from a visited network if one of the peer user terminals is roaming; and

prior notification from the packet switched access network used by one of the peer user terminals.

10. A Session Initiation Protocol (SIP) server for use in an Internet Protocol (IP) Multimedia Core Network Subsystem, the server comprising:

means for receiving an INVITE request from a dual-mode user terminal, over an IP based packet switched domain, initiating a packet switched session;

means for determining that the packet switched session requires setting up of at least one circuit switched conversational bearer over a circuit switched access network serving the dual-mode user terminal;

means for sending a notification from the SIP server to a gateway server that one or more conversational bearers must be established, the gateway server terminating the circuit switched session within the system; and

means for receiving a callback telephone number from the gateway server and for forwarding the callback number to the dual-mode terminal.

11. A gateway server for providing an interface between a circuit switched access network and a packet switched network, the gateway server having an interface towards a Session Initiation Protocol (SIP) server of an Internet Protocol (IP) Multimedia Core Network Subsystem, said gateway server comprising:

means for receiving from the SIP server, signaling instructing the setting up of a circuit switched call over the circuit switched access network with a dual-mode user terminal;

means for sending a callback number to the dual-mode user terminal in response to receipt of the signaling; and

means for terminating a call made to the callback number by the dual-mode user terminal.