HP-UX Java SIP Stack Programmer's Guide (February 2008)

Figure 1-3 Call Setup Using a Proxy server
The following messages are exchanged while UA1 uses a proxy server to connect to
UA2:
1. UA1 sends an INVITE request to UA2. This request is first sent to the proxy server
and then to UA2.
2. The proxy server responds with a 100–Trying message to UA1.
3. The proxy server looks up user2's current location in a location service.
4. The location service returns user2's current location.
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
UA2.
6. UA2 responds with a 100–Trying message to the proxy server.
7. The UA2 responds with a 180–Ringing response to the proxy server.
8. The proxy server forwards the 180–Ringing response back to UA1.
9. UA2 sends a 200–OK response, when the call is accepted by the user (for example,
by picking up the handset), .
Call Flow Scenarios 31