A method of establishing a session of a real-time media communication service in a communication system comprising at least two separately administered subsystems (31, 32), comprising: receiving, in a server, a request (3.2) relating to the session from an originating subscriber, wherein the request comprises identification information on a terminating subscriber; detecting (3.6), in the server, that the terminating subscriber and the originating subscriber belong to different subsystems (31, 32); maintaining, in the server, transit server information that associates a group of subscribers with a transit server, wherein the association is based on identification information of subscriber and said transit server has access to a subscriber database of a subsystem of the associated terminating subscriber; determining, in the server, in response to detecting that the terminating subscriber and the originating subscriber belong to different subsystems (31, 32), with the identification information, the transit server associated with the terminating subscriber querying subscriber information of the terminating subscriber related to the requested session with a first control message (3.7) from the server to the transit server, wherein the subscriber information of the terminating subscriber comprises identity of a user control server of the terminating user and the first control message (3.7) comprises parameters of the requested session; retrieving, in response to a received control message (3.7), the subscriber information from the subscriber database of the subsystem of the terminating subscriber; returning a second control message (3.14) comprising queried session related subscriber information; wherein the request (3.2) is a request for session initiation and that the method further comprises initiating (3.15), in the server, the requested session according to the queried session related subscriber information by sending a request (3.15) to the user control server of the terminating user, wherein said session of the real-time media communication service is established according to a Session Initiation Protocol, SIP, and the real-time media communication service related information is carried in a payload of Session Initiation Protocol messages (3.7, 3.14, 3.15). receiving, in a server, a request (3.2) relating to the session from an originating subscriber, wherein the request comprises identification information on a terminating subscriber; detecting (3.6), in the server, that the terminating subscriber and the originating subscriber belong to different subsystems (31, 32); maintaining, in the server, transit server information that associates a group of subscribers with a transit server, wherein the association is based on identification information of subscriber and said transit server has access to a subscriber database of a subsystem of the associated terminating subscriber; determining, in the server, in response to detecting that the terminating subscriber and the originating subscriber belong to different subsystems (31, 32), with the identification information, the transit server associated with the terminating subscriber querying subscriber information of the terminating subscriber related to the requested session with a first control message (3.7) from the server to the transit server, wherein the subscriber information of the terminating subscriber comprises identity of a user control server of the terminating user and the first control message (3.7) comprises parameters of the requested session; retrieving, in response to a received control message (3.7), the subscriber information from the subscriber database of the subsystem of the terminating subscriber; returning a second control message (3.14) comprising queried session related subscriber information; wherein the request (3.2) is a request for session initiation and that the method further comprises initiating (3.15), in the server, the requested session according to the queried session related subscriber information by sending a request (3.15) to the user control server of the terminating user, wherein said session of the real-time media communication service is established according to a Session Initiation Protocol, SIP, and the real-time media communication service related information is carried in a payload of Session Initiation Protocol messages (3.7, 3.14, 3.15).