There is provided a method of operating an IP Multimedia Subsystem Access Gateway (IMS-AGW) in order to support the transmission of media to and from an endpoint, using an IMS, wherein the endpoint implements an alternative media transport protocol other than Real-time Transport Protocol (RTP). The method comprises the IMS-AGW receiving instructions from an IMS Application Level Gateway (IMS-ALG) for an alternative media transport protocol connection with the endpoint and a RTP connection within the IMS, the instructions including more than one RTP payload type number and a dynamic binding of each of the more than one RTP payload type number to a statically defined codec identifier. The IMS-AGW stores the dynamic binding. Then, when translating between alternative media transport protocol packets and RTP packets, the IMS-AGW uses the stored binding to map between an RTP payload type number and a codec identifier used by the alternative media transport protocol.