A method and apparatus for handling a Session Initiation Protocol communication in an IP Multimedia Subsystem (IMS) network. A Proxy Call Session Control Function (3) receives a Session Initiation Protocol message sent from a trusted remote network. The P-CSCF (3) adds to the message a further header, which identifies a Public User Identity of a trusted entity located in the remote network served by a Serving Call Session Control Function (7) in the IMS network. The message is then sent to the S-CSCF (7). The S-CSCF (7), and any other node that the message is sent to, knows from the presence of the furtherheader to use the Public User Identity of the trusted entity to determine the served user rather than the P-Asserted Identity contained in the SIP message.