Asterisk Open Source PBX
Asterisk is a complete PBX in software. It runs on Linux and provides all of the features you would expect from a PBX and more. Asterisk does voice over IP in many protocols, and can interoperate with almost all standards-based telephony equipment using relatively inexpensive hardware.
http://www.asterisk.org/

Brekeke VOIP Software Developers
Brekeke Software, Inc. is a telephony software developer, founded in 2002 in San Mateo, California, specializing in SIP VoIP telephony products. Our main market focus has always been to help implement new VoIP systems in small to mid-sized companies who haven't yet been introduced to the many benefits of VoIP technology.
http://www.brekeke.com/en/company/company_en.html

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