Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Figure 7.1 Call-forward, no-answer service implemented by the proxy server

Called User Agent Implementation

Figure 7.2 shows how the same feature can be implemented in the called SIP
phone. In this case, the ring-no-answer timer is started in the called SIP user
agent. When the timer expires, the phone sends a redirection response:

302 Moved Temporarily

Contact: <sips:[email protected]>

This causes the calling SIP phone to generate an ACKto the called SIP phone
then generate a new INVITEdirectly to the voicemail server, which then
answers, plays a prompt, and records a message.

Proxy Server

1 INVITE

3 100 Trying

6 180 Ringing

2 INVITE

4 100 Trying

5 180 Ringing

7 CANCEL Proxy stops

phone ringing using

CANCEL then forks

INVITE.

8 200 OK

9 487 Request Cancelled

10 ACK

11 INVITE

12 200 OK

15 ACK

Media Session Established

13 200 OK

14 ACK

SIP Caller SIP Phone Voicemail Server

SIP Service Creation 137