Scenario 2 INVITE without SDP
For an offerless call flow, the system creates a media session when the offer comes in a reliable provisional or final response. The Access Session Border Controller (A-SBC) applies the codec policy and sends the egress offer to the calling UE.
When the A-SBC receives an answer in either a PRACK or ACK message, the A-SBC compares the offer and answer. An answer containing the codec added in the egress offer causes the A-SBC to invoke the Transcoding Session Border Controller (T-SBC) and create a standalone UAC dialog, just as in Scenario 1.
Because the A-SBC advertised its media address in the egress offer to the calling UE, the UAC dialog uses the original media session.
