Prioritising calls

1. A gateway operable in an IP Multimedia Subsystem (IMS) network, the gateway being configured to:

receive a first call setup request to set up a first call, the first call setup request being received from an emergency service, the first call being between the emergency service and a user terminal, the received first call setup request comprising a first called party identifier;

determine whether the called party identifier is of a first type or of a second, different type, wherein the first type of called party identifier comprises a dialable number temporarily allocated to the user terminal for use in receiving a call from the emergency service, wherein the dialable number is from a pool of dialable numbers reserved for emergency calls involving the emergency service;

in response to determining that the first called party identifier is of the first type, transmit priority indication data to a Call Session Control Function (CSCF) in the IMS network in order to cause the CSCF to prioritize handling of the first call between the emergency service and the user terminal;

receive a second call setup request to set up a second call, the second call setup request comprising a second, different, called party identifier;

determine whether the second called party identifier is of the first type or of the second type, wherein the second type of called party identifier does not comprise a dialable number temporarily allocated for use in receiving a call from the emergency service; and

not transmit the priority indication data in relation to the second call in response to determining that the second called party identifier is of the second type.

2. The gateway of claim 1 , wherein the received first call setup request does not comprise the priority indication data.

3. The gateway of claim 1 , wherein the received first call setup request is in accordance with a first protocol that does not allow the priority indication data to be comprised in the received first call setup request.

4. The gateway of claim 3 , wherein the first protocol is used to set up calls involving a public switched telephone network.

5. The gateway of claim 3 , wherein the gateway is configured to transmit a third call setup request in accordance with a second, different protocol.

6. The gateway of claim 5 , wherein the second protocol is a session initiation protocol.

7. The gateway of claim 5 , wherein the transmitted third call setup request comprises the first called party identifier.

8. The gateway of claim 5 , wherein the transmitted third call setup request comprises the priority indication data.

9. The gateway of claim 1 , wherein the gateway comprises:

an internet protocol private branch exchange;

a signaling gateway;

a media gateway;

a media gateway control function;

an emergency call server;

a breakout gateway control function; and/or

a Point of Interface.

10. The gateway of claim 1 , wherein the gateway does not comprise:

an emergency call session control function;

a proxy call session control function;

a serving call session control function; and/or

an interrogating call session control function.

11. The gateway of claim 1 , wherein the priority indication data is operable to cause the CSCF, when prioritizing the handling of the call between the emergency service and the user terminal, to circumvent:

verifying an authentication state of the user terminal:

verifying an authorization state of the user terminal;

verifying a registration state of the user terminal;

verifying whether or not a supplementary service is configured for the user terminal; and/or

verifying credit and billing information associated with the user terminal.

12. The gateway of claim 1 , wherein the temporary association of the dialable number and the user terminal is on the basis of:

a time period;

an authorization state associated with the user terminal; and/or

an association between the user terminal and a radio access network.

13. The gateway of claim 1 , wherein the dialable number is from a pool of dialable numbers reserved for calls involving user terminals without credentials.

14. The gateway of claim 1 , wherein the dialable number is from a pool of dialable numbers reserved for calls involving user terminals with credentials.

15. The gateway of claim 1 , wherein the CSCF comprises an emergency CSCF.

16. A method of controlling a gateway in an IP Multimedia Subsystem (IMS) network, the method comprising, at the gateway:

receiving a first call setup request to set up a first call, the first call setup request being received from an emergency service, the first call being between the emergency service and a user terminal, the received first call setup request comprising a first called party identifier;

determining whether the called party identifier is of a first type or of a second, different type, wherein the first type of called party identifier comprises a dialable number temporarily allocated to the user terminal for use in receiving a call from the emergency service, wherein the dialable number is from a pool of dialable numbers reserved for emergency calls involving the emergency service;

in response to determining that the first called party identifier is of the first type, transmitting priority indication data to a Call Session Control Function (CSCF) in the IMS network in order to cause the CSCF to prioritize handling of the first call between the emergency service and the user terminal;

receiving a second call setup request to set up a second call, the second call setup request comprising a second, different, called party identifier;

determine whether the second called party identifier is of the first type or of the second type, wherein the second type of called party identifier does not comprise a dialable number temporarily allocated for use in receiving a call from the emergency service; and

not transmitting the priority indication data in relation to the second call in response to determining that the second called party identifier is of the second type.

17. A non-transitory computer-readable storage medium comprising a set of computer-readable instructions stored thereon, which, when executed by a gateway, cause the gateway to perform a method in an IP Multimedia Subsystem (IMS) network, the method comprising, at the gateway:

receiving a first call setup request to set up a first call, the first call setup request being received from an emergency service, the first call being between the emergency service and a user terminal, the received first call setup request comprising a first called party identifier;

determining whether the called party identifier is of a first type or of a second, different type, wherein the first type of called party identifier comprises a dialable number temporarily allocated to the user terminal for use in receiving a call from the emergency service, wherein the dialable number is from a pool of dialable numbers reserved for emergency calls involving the emergency service;

in response to determining that the first called party identifier is of the first type, transmitting priority indication data to a Call Session Control Function (CSCF) in the IMS network in order to cause the CSCF to prioritize handling of the first call between the emergency service and the user terminal;

receiving a second call setup request to set up a second call, the second call setup request comprising a second, different, called party identifier;

determine whether the second called party identifier is of the first type or of the second type, wherein the second type of called party identifier does not comprise a dialable number temporarily allocated for use in receiving a call from the emergency service; and

not transmitting the priority indication data in relation to the second call in response to determining that the second called party identifier is of the second type.