Because they are generally less expensive, open source products may become attractive to more corporate users as their budgets are cut, laying the groundwork for a growth spurt, according to the recent study by Eastern Management Group.

"The price of [open-source] PBXs is so low as to attract more than passing attention," says John Malone, president and CEO of the group.

If the study is accurate, it means that open source vendors as a group are selling more PBX phone lines than any single PBX vendor that makes the usual list of market leaders.

The study relies on market models and more than 7,000 interviews with IT executives, vendors and resellers and claims to find 2.85 million VoIP endpoints that have gone uncounted in traditional tallies, including its own.

Via surveys of IT executives as well as open-source PBX vendors and VARS, the study concluded that use of this gear grew 40% last year and will increase 40% in 2009, partially fueled by tighter IT budgets.