A method of managing Session Initiation Protocol, SIP, -based sessions in a first device, comprising: activating a first communication session for a first communication service of an Internet Protocol Multimedia Subsystem, IMS, application for a first media format at a first user's first request for the first communication service (S101, S403); assigning a session mapping identifier to the first communication session (S105); sending a first SIP session negotiation request including the assigned session mapping identifier to a second device of a second user (S109, S409); receiving a second request for a second communication service of the IMS application from the first user, wherein the second communication service is different from the first communication service and is for a second media format different from the first media format, wherein the second request identifies the second user or device; in response to the second request, activating a second communication session (S201, S419) for the second communication service; assigning the same session mapping identifier to the second communication session (S207); sending a second SIP session negotiation request including the same session mapping identifier to the second device (S213, S423); and establishing the second communication session with the second device using the same session mapping identifier; wherein establishing the second communication session includes mapping, at the second device, the second SIP session negotiation request to the IMS application of the first communication session by using the session mapping identifier; characterized in that the assigned session mapping identifier is inserted into an Application-ID header field of the first SIP session negotiation request; in that the same session mapping identifier is inserted into an Application-ID header field of the second SIP session negotiation request; and in that the session mapping identifier is configured as a 16-nary random number. activating a first communication session for a first communication service of an Internet Protocol Multimedia Subsystem, IMS, application for a first media format at a first user's first request for the first communication service (S101, S403); assigning a session mapping identifier to the first communication session (S105); sending a first SIP session negotiation request including the assigned session mapping identifier to a second device of a second user (S109, S409); receiving a second request for a second communication service of the IMS application from the first user, wherein the second communication service is different from the first communication service and is for a second media format different from the first media format, wherein the second request identifies the second user or device; in response to the second request, activating a second communication session (S201, S419) for the second communication service; assigning the same session mapping identifier to the second communication session (S207); sending a second SIP session negotiation request including the same session mapping identifier to the second device (S213, S423); and establishing the second communication session with the second device using the same session mapping identifier; wherein establishing the second communication session includes mapping, at the second device, the second SIP session negotiation request to the IMS application of the first communication session by using the session mapping identifier; characterized in that the assigned session mapping identifier is inserted into an Application-ID header field of the first SIP session negotiation request; in that the same session mapping identifier is inserted into an Application-ID header field of the second SIP session negotiation request; and in that the session mapping identifier is configured as a 16-nary random number. <EOS>