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What is Voicemail?

Voicemail is a messaging service normally associated with telephony applications. It can be applied as a service in a network (as provided by mobile phone providers), in a separate device such as a home answering machine, or integrated in a telephony device such as an enterprise PBX or key system.

Voicemail involves call forwarding no answer/busy/unavailable to a storage device which plays a customizable greeting. The end user is then alerted by some means that a message is waiting, and can then retrieve the message by dialing into the system.

In SIP terms, the call forwarding is simple, with either a proxy forwarding or endpoint redirection (3xx response) used to send the call to the voicemail server. However, some kind of SIP extension is needed to signify to the voicemail system which mailbox to use—that is, which greeting to play and where to store and save the recorded message.

We have two ways to do this

1. Using a SIP header field extension

2. Using the Request-URI to signal this information

For a voicemail system at sip:voicemail.example.com, which is being used to supply voicemail for sip:[email protected], the Request-URI of the INVITE when it is forwarded to the voicemail server could look like:

sip:voicemail.example.com;target=sip:[email protected];cause=486
In this way, the Request-URI carries the mailbox identifier as well as the motive the call is being forwarded to the voicemail.

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