Asterisk is an Open Source PBX and telephony development platform that can both replace a conventional PBX and act as a platform for developing custom telephony applications for delivering dynamic content over a telephone similarly to how one can deliver dynamic content through a web browser using CGI and a web server.
Asterisk talks to a variety of telephony hardware including BRI, PRI, POTS, and IP telephony clients using the Inter-Asterisk eXchange protocol (e.g. gnophone or miniphone).
For more information and a current list of supported hardware, see www.asteriskpbx.com.
