User's Manual

Chapter 21 Voice 186
1 A se
nds a SIP INVITE request to B. This message is an invitation for B to participate in a SIP
telephone call.
2 B se
nds a response indicating that the telephone is ringing.
3 B
sends an OK response after the call is answered.
4 A
then sends an ACK message to acknowledge that B has answered the call.
5 No
w A and B exchange voice media (talk).
6 After tal
king, A hangs up and sends a BYE request.
7 B r
eplies with an OK response confirming receipt of the BYE request and the call is terminated.
SIP Call Progression Through Proxy Servers
Usually, the SIP UAC sets up a phone call by sending a request to the SIP proxy server. Then, the
p
roxy server looks up the destination to which the call should be forwarded (according to the URI
requested by the SIP UAC). The request may be forwarded to more than one proxy server before
arriving at its destination.
The response to the request goes to all the proxy servers through which the request passed, in
r
everse sequence. Once the session is set up, session traffic is sent between the UAs directly,
bypassing all the proxy servers in between.
3. OK
4. ACK
5.Dialogue (voice traffic)
6. BYE
7. OK
Table 104 SIP Call Progression (continued)
A B