HP-UX C SIP Stack Programmer's Guide (Novembery 2007)

Session Initiation Protocol (SIP) 13
Entity Interaction
5. The Proxy server decides to Proxy the call and creates a new
INVITE transaction based on the original INVITE message,
but with the Request-URI in the start line changed to
[e-mail]. The Proxy server sends this request to the
UA2.
6. The UA2 responds first with a 100 (Trying).
7. The UA2 responds with a 180 (Ringing) response.
8. The Proxy server forwards the 180 (Ringing) response back to
the UA1.
9. When the call is accepted by the user (for example, by picking
up the handset) UA2 sends a 200 (OK) response. In this
example, UA2 inserts a Contact header into the response with
the value [e-mail]. Further SIP communication will
be sent directly to it and not via the Proxy server.
10. The Proxy forwards the 200 (OK) response back to the calling
UAC.
11. The calling UA sends an ACK directly to UA2 at the lab
(according to the Contact header it found in the 200 (OK)
response).