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

The caller then sends an INVITEto the first URI. After the ring-no-answer
timer expires in the calling user agent, the caller sends a CANCEL, then sends a
new INVITEto the voicemail server. This second Contactheader shows the
use of feature tags (covered in Chapter 8, “User Preferences”) in specifying
that the URI is that of a voicemail server.

Figure 7.3 Call-forward, no-answer service implemented by the calling user agent

SIP Caller Proxy Server

1 INVITE

2 100 Trying

4 ACK

5 INVITE

Calling SIP Phone

cancels call to

phone then sends

INVITE directly to

voicemail server.

Media Session Established

9 200 OK

10 487 Request Cancelled

6 100 Trying

8 CANCEL

11 ACK

12 INVITE

SIP Phone Voicemail Server

Server redirects

with Contact URLs

of both the SIP

phone and

voicemail server.

3 302 Moved Temporarily

7 180 Ringing

13 200 OK

14 ACK

SIP Service Creation 139