Method, system and entity for a media transfer session in an IMS infrastructure

There is provided a mechanism for having an operator of an IP network that is applied for transferring the media transfer session stream for an IP Multimedia Subsystem, IMS, call to relate this stream to a specific operator of the home-network of the calling party User Equipment. By having, e.g., a Session Border Gateway, SBG, insert a specific signature, associated with the operator sourcing the media transfer session for the calling party UE, into the path carrying the media transfer session stream, the operator of the IP transport network is, by discovery of the specific signature, and its characteristics enabled to discover the operator sourcing the media transfer session.