A solution for establishing a session of a real-time media communication service in a communication system comprising at least two separately administered subsystems. The session establishment comprises receiving a request for session initiation, querying subscriber information related to the requested session, and initiating the session according to the queried subscriber information. The invented method comprises detecting that the terminating subscriber does not belong to the same subsystem as the originating subscriber, determining a defined transit server associated with the terminating subscriber, said transit server having access to a subscriber database of the subsystem of the terminating subscriber, and querying subscriber information related to the requested session with a first control message comprising parameters of the requested session to the transit server. The solution allows a connection to be established between users of a real-time media communication service for subscribers of separate administrative subsystems so that operators of each subsystem may possess full control of their own network elements, and the internal connection establishment procedures need minimal alterations for the functionality.