Diagram illustrate a successful SIP IP phone-to-SIP IP phone call.
INVITE: UA-A to UA-B
100 Trying: UA-B to UA-A
UA-B sends a 100 Trying response to UA-A so that it stops retransmission of INVITE message.
180 Ringing: UA-B to UA-A
UA-B sends a 180 Ringing response to the UA-A. At this point UA-B starts ringing.
200 OK: UA-B to UA-A
UA-B sends a 200 OK response to UA-A. The response notifies UA-A that UA-B has answered the phone.
ACK: UA-A to UA-B
UA-A sends an ACK to UA-B. The ACK confirms that UA-A has received the 200 OK response from UA-B.
Media Session: UA-A and UA-B
A media session is considered established when the INVITE, 200 OK, and ACK messages have been exchanged between the UA-A and the UA-B.
BYE: UA-B to UA-A
The request indicates that UA-B wants to release the call. Because it is UA-B that wants to terminate the call, the Request-URI field is now replaced with UA-A SIP URL and the From field contains UA-B SIP URL.
200 OK: UA-A to UA-B
This response notifies UA-B that UA-A has received the BYE request.