Asterisk runs on a wide variety of operating systems, including Windows, Linux, Mac OS X, OpenBSD, FreeBSD and Sun Solaris. It includes the high-end features of proprietary PBXs and operates on off-the-shelf software. 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 (http://www.computerworld.com/action/article.do?command=viewArticleBasic&articleId=9003067).

The trend to open-source technologies is somewhat limited now to companies that have relatively simple needs, have some technical expertise on staff or both, according to analysts. But the movement is clearly gaining momentum, with companies either replacing existing systems with open-source technologies or choosing open-source over proprietary products for new installations.

For example, Bill Ciminelli, vice president of network development and services at American Fiber Systems Inc. in Rochester, N.Y., is in the final stages of converting his company's PBX systems (one Nortel and a shared NEC system) to an open-source-based Asterisk system.

"If someone had presented the idea of an open-source system to me 18 months ago, I would have said no way," he says. "Now if someone comes to me with a [technology], I ask first if there's an open-source option."

What it came down to in the end was price for performance. Ciminelli figures that he would have to pay at least three times as much for the same number of connections on a Cisco system as he has on the Asterisk solution.