A.INVITE,OPTIONS,ACK,BYE,CANCEL,REGISTER
B.INVITE,ACK,OPTIONS,BYE,CANCEL,REGISTER
C.INVITE,ACK,OPTIONS,CANCEL,BYE,REGISTER
D.REGISTER,INVITE,ACK,OPTIONS,CANCEL,BYE
A.IAM=INVITE
B.ACM=180 RINGING
C.ANM=200 OK
D.REL=BYE
E.RLC=200OK(for BYE)
A.pending
B.complete
C.ack
D.request