But there is an alternative, as thousands of businesses and network administrators have discovered. The open-source Asterisk PBX has been gaining a big following, offering surprisingly powerful telephony features on inexpensive hardware. Not only has it been saving companies money, but it has also been able to integrate telephony with network applications in ways that previously might not have been possible.

But Asterisk isn't for everyone. And there are issues you need to confront if you plan a move to Asterisk. So here, in a nutshell, is what you need to know about Asterisk, along with advice from those who have already deployed it.

What is Asterisk?

Let's start with the basics: Asterisk is open-source PBX software that runs on a wide variety of operating systems, including Windows, Linux, Mac OS X, OpenBSD, FreeBSD and Sun Solaris. It can run on inexpensive, off-the-shelf hardware, and it includes high-end features such as interactive voice response, voice mail, conference calling, and automatic call distribution and routing that have until now only been available on proprietary PBXs.

It's also exceedingly flexible. New functions can be created by writing scripts in Asterisk's language, by writing modules in C, and by writing scripts in Perl or other languages.

Particularly important is that it handles VoIP calls and works with a variety of VoIP protocols, including the Session Initiation Protocol (SIP) and H.323. It also functions as a gateway between IP phones and the public switched telephone network.

All this means that it can be used to create powerful, programmable PBXs at a low cost, says Joshua Stephens, CEO of Switchvox, a San Diego-based integrator and provider of PBX systems, including many built using Asterisk.

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