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

Table 12.1 (continued)

REASON FOR REDIRECT VALUE

Unconditional 302

Deflection During Alerting 487

Deflection Immediate Response 480

Mobile Subscriber Not Reachable 503

Even more comprehensive information (such as when the call has been
retargeted multiple times) can be conveyed using the Request History Infor-
mation [6]. Comprehensive retargeting information can be used for compati-
bility with legacy TDM systems, to preserve their complex functionality.
The limitation for using rich reason codes or the Request History Informa-
tion is that most SIP proxies need to be designed to understand these exten-
sions to SIP and must be programmed to support the behavior required for
such service.
An example of forwarding to voicemail with the reason indication is given
here in the scenario where the endpoint forwards on busy to voicemail. This
example shows the service based on a SIP proxy server but is also of interest
for peer-to-peer (P2P) SIP, since the forwarding function is performed by the
target endpoint. So, for certain causes (such as busy or no answer), this exam-
ple can also work without the SIP server in a pure P2P fashion. The flow dia-
gram is shown in Figure 12.3.

Figure 12.3 Flow diagram for voice message deposit with indication of cause

Alice Proxy Bob Voicemail

1 INVITE

2 INVITE

3 302 MOVED

6 ACK

7 INVITE

4 302 MOVED

5 MOVED

Voicemail and Universal Messaging 215