Notifying changes in radio access technology

An apparatus for use in a network node in a telecommunications network comprising an IP Multimedia Subsystem, IMS, the apparatus comprising:

• processor circuitry;
• a storage unit storing instructions executable by said processor circuitry, a transmitter and a receiver, whereby the apparatus is operative to:
• receive a SIP REGISTER message including a capability indication from a Call Session Control Function, CSCF, that the CSCF supports RAT change notifications for a user equipment;
• and
• provide to the CSCF a SIP SUBSCRIBE message containing a requirement indication, which indicates that the network node requires RAT change notifications for the user equipment.

A method of notifying changes in Radio Access Technology, RAT, to one or more network nodes in a communications network, comprising an Internet Protocol Multimedia Subsystem, IMS, of a change in Radio Access Technology, RAT, used by a user equipment for a communications session, the method comprising:

• at a Call Session Control Function, CSCF:
• prior to set-up of the communications session involving a user equipment:
• indicating by the CSCF to a Policy Charging and Rules Function, PCRF, that the CSCF requires RAT change notifications for the user equipment;
• in response to reception of a SIP REGISTER message from the user equipment, including a capability indication that the CSCF supports RAT change notifications in the SIP REGISTER message;
• forwarding the SIP REGISTER message to one or more network nodes;
• receiving from the one or more network nodes a SIP SUBSCRIBE message containing a requirement indication, which indicates that the network node requires RAT change notifications associated with the user equipment;
• on receipt of a RAT change notification by the CSCF during the communications session:
• propagating the RAT change notification for the user equipment using a SIP NOTIFY message to each of the one or more network nodes from which a requirement indication has been received.