Infonetics did find that the worldwide growth of PBX sales did grow 1% last year, but that growth figure was also a drop from the 7% growth it reported the year before. "I would say it was the economy," he says, not the incursion of open source alternatives that accounts for the decline.

Machowinski said open source PBX vendors should be willing to claim such numbers. "If this is really the case, why won't companies like Digium report what their line shipments are?" he says. "You'd think that would be something they'd like to provide."

Bill Miller, vice president of product management for Digium, which sells commercial Asterisk products and peripherals, says all the company knows for sure about the software s the number of downloads, but not whether they are actually used.

But he has estimates based on downloads and the purchase of peripherals such as line cards and codecs for the systems that give an indication of what I actually in use. He puts that number at 2.7 million lines installed in North America last year, which is more than Eastern Management's estimate. He estimates that just 5% of the downloaded Asterisk PBX software is in production use at businesses.

Machowinski says that someone could install a free open source IP PBX but that doesn't necessarily mean they would have bought one from a proprietary vendor if there were no open source alternatives. So the 18% that use open source might not represent a loss to the leading non-open-source PBX vendors. "Even if it's 18%, does it displace 18% of the market?" he says.