message containing its own sdp 17 the application server as 2 sends the update message to the soft switch 2 which then returns the update message containing a latest sdp of the callee to the soft switch 1 18 the soft switch 1 returns the update message containing a sdp of the callee to the application server as 1 19 the application server as 1 sends the re invite message containing a sdp of the callee to the caller and performs the session negotiation with a caller so as to update the sdp of the callee at the caller side 20 the caller performs the update of the sdp of the callee changes the sdp of the original connected mrs of the caller to the latest sdp of the callee and after the success of the update returns the 200 ok response message containing its own sdp to the application server as 1 21 the application server as 1 returns the 200 ok response message containing a sdp of the caller to the soft switch 1 which then sends the message to the soft switch 2 22 the soft switch 2 returns the 200 ok response message containing a sdp of the caller to the application server as 2 23 the application server as 2 sends the final ack message containing a sdp of the caller to the callee and till.