From ITBusiness.ca:

Private branch exchanges (PBXs) with open source telephony software may cost less than voice products from major manufacturers, but one industry observer warns companies who buy cheaper products will usually get fewer features.

Equipment manufacturers, resellers and end-users are buying open source software because they hear it will save them money, said Kevin Fleming, senior software engineer for Digium Inc. of Huntsville, Ala., which makes Asterisk, a Linux-based open source PBX operating system.

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