A method of processing session initiation protocol (SIP) call flows, the method comprising, at a back-to-back user agent (B2BUA) configuring a media user agent (UA) layer to interface between a SIP UA layer and a Media B2BUA layer. for call flow operation in the SIP UA layer to Media B2B UA layer direction, the configuring comprises, at the media UA layer first mapping each message received from the SIP UA layer comprised of one of a plurality of SIP layer message types into one of a SDP offer message type, a SDP answer message type and a third message type different from the SDP offer message and SDP answer types, and passing each first mapped message to the Media B2B UA layer for processing.