* @desc Originating Leg With SDP Negotiation in INVITE/200 OK.
* On successful call setup, the P-CSCF should derive from the SDP offer and answer, descriptions of the Service Data Flow.
* These are pushed towards EPC as request for creation of adequate bearers. EPC creates based on the EPC's operator policies the bearers for media.
* When transporting media, the EPC will employ the respective bearer's characteristics. Media transport is possible only after the successful establishment of the session.
* Media negotiation happens during INVITE/200 OK (UE A sends SDP-offer, UE B responds with SDP-answer)