HP FlexNetwork MSR Series Configuration Manual page 146

Figure 36 Message exchange for a UA to register with a Registrar
Call setup
SIP operates in the Client/Server mode and sets up calls through communication between UA and
proxy server.
Figure 37 Network diagram
In the above figure, Telephone A will call Telephone B, and Router A and Router B work as SIP
endpoints (UAs).
The following is the procedure for connecting a call from Telephone A to Telephone B:
1. Telephone A sends the number of Telephone B.
2. Upon receipt of the call, Router A sends a session request (INVITE) to the proxy server.
3. The proxy server consults its database for information corresponding to the number of
Telephone B. If such information is available, it forwards the request to Router B.
4. Router B, after receiving the request, responds to the proxy server and makes Telephone B ring
if Telephone B is available.
5. The proxy server forwards the response to Router A. The response discussed here includes
two provisional response messages (100 Trying and 180 Ringing) and one success response
(200 OK).
Figure 38
illustrates the complete call setup procedure.
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