System information

Using Asterisk As a Standalone Voicemail Server
In a traditional telecom environment, the voicemail server was typically a standalone
unit (provided either as a separate server altogether, or as an add-in card to the system).
Very few PBXs had fully integrated voicemail (in the sense that voicemail was an integral
part of the PBX rather than a peripheral device).
Asterisk is quite capable of serving as a standalone voicemail system. The two most
common reasons one might want to do this are:
1. If you are building a large, centralized system and have several servers each pro-
viding a specific function (proxy server, media gateway, voicemail, conferencing,
etc.)
2. If you wish to replace the voicemail system on a traditional PBX with an Asterisk
voicemail
Asterisk can serve in either of these roles.
Integrating Asterisk into a SIP Environment As a Standalone
Voicemail Server
If you want to have Asterisk act as a dedicated voicemail server (i.e., with no sets reg-
istered to it and no other types of calls passing through it), the process from the dialplan
perspective is quite simple. Getting message waiting to work can be a bit more difficult,
though.
Let’s start with a quick diagram. Figure 8-1 shows an overly simplified example of a
typical SIP enterprise environment. We don’t even have an Asterisk server in there
(other than for the voicemail), in order to give you a generic representation of how
Asterisk could serve as a standalone voicemail server in an otherwise non-Asterisk
environment.
Unfortunately, Asterisk cannot send message notification to an endpoint if it doesn’t
know where that endpoint is. In a typical Asterisk system, where set registration and
voicemail are handled on the same machine, this is never a problem, since Asterisk
knows where the sets are. But in an environment where the sets are not registered to
Asterisk, this can become a complex problem.
There are several solutions on the Internet that recommend using the externnotify
option in voicemail.conf, triggering an external script whenever a message is left in a
mailbox (or deleted). While we can’t say that’s a bad approach, we find it a bit kludgy,
and it requires the administrator to understand how to write an external script or pro-
gram to handle the actual passing of the message.
174 | Chapter 8: Voicemail