Section 3.5. Key Issues: Linux as a PBX

Asterisk is an open source PBX in software that runs on Linux, BSD, and Mac OS X. It's a great test environment for evaluating VoIP; it's also a solid solution for production systems
Because Asterisk was written in C on Linux from the ground up, with many POSIX conventions at its heart, a duty-ready version of Asterisk for Windows doesn't exist.
Digium, Quicknet, and others make interface cards that allow Linux softPBX servers to connect to the PSTN using POTS and T1/E1 lines
The Asterisk CLI is an administrative interface that allows programming of extensions and monitoring of system activity
The Asterisk Management socket API allows the Astman application to monitor calls in progress on an Asterisk softPBX. It's also possible to use the API to build your own applications
Asterisk, like many softPBX systems, refers to each leg of a voice call as a channel
Asterisk is commercially maintained by Digium, Inc., and supported by Digium and other consulting firms