In current multimedia communication systems using session initiation protocols such as SIP, a service change (e.g. adding a new media type to an existing multimedia conversation) entails significant delays and processor load in both clients and server. The current invention solves this by separating session signaling and media control signaling in different signaling channels (141,142) and by eliminating the need to re-establish SIP sessions for each service change. The application server (120) maintains a list of all media types supported by each multimedia client (110) involved in a multimedia conversation. Each multimedia client (110) requesting to send one or several media streams with different media types to one or several other multimedia client(s) negotiates with the application server (120) only. The inventive concept significantly reduces networks delays and speeds up the service change as perceived by the user. The invention is of interest for various multimedia conferencing applications.