A method and IP Multimedia Subsystem (IMS) gateway for establishing a unicast media session between a media receiver and a media source in an IMS network via a Network Address Translation (NAT) function acting on behalf of the media receiver. A media source controller in the IMS sends a Session Initiation Protocol (SIP) INVITE message to the media receiver to identify the media source. The media receiver obtains NAT bindings for the unicast media session, and the session is established between the media receiver and the media source.