Volte MO Call 信令

1、MO call flow:
1.1、MO UE sends INVITE message to start VoLTE call.
1.2、MO UE reveives 100 Trying from network.
1.3、MO UE receives 183 Session Progress.
1.4、MO UE sends PRACK to indicate having received 183.
1.5、MO UE receives 200 OK(PRACK).
1.6、MO UE sends UPDATE to handle precondition process.
1.7、MO UE receives 200 OK(UPDATE).
1.8、MO UE receives 180 Ringing from network.
1.9、MO UE sends PRACK to indicate having received 180.
1.10、MO UE receives 200 OK(PRACK).
1.11、MO UE receives 200 OK(INVITE) which means MT UE has answered the call.
1.12、MO UE sends ACK.
1.13、MO UE sends Bye to end call.
1.14、MO UE receives 200 OK(Bye)