"Asterisk is the worlds leading open source telephony engine and tool kit. Offering flexibility unheard of in the world of proprietary communications, Asterisk empowers developers and integrators to create advanced communication solutions...for free." - http://www.asterisk.org/
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Its a great tool! I used one small Linux box with asterisk to replace six or seven little one shot proprietary apps. (Teleco vendors are *by far* the worst in the industry and it nicely integrated with the proprietary stuff I decided to keep..
The configuration is remarkably simple (for a "PBX and so much more")
OSS Ping
The last switch I even did any work on was a Definity. Ran UNIX of course, still didn’t have the flexibility this *strisk has.