= an open source implementation of a PBX.
See: Open Telephony
"the telecommunication industry had an interoperable and very interchangeable open standard for telephone exchanges. The common protocol was English delivered by many operators sitting in large rooms with large patch panels. With the advent of electronics, manufacturers developed their own proprietary telephone exchange technologies and protocols. In 1992, the industry developed CSTA (Computer Supported Telecommunications Applications) as a standard abstraction layer for telecommunications applications that was independent of the underlying protocols. It became an OSI standard in July 2000 and is still maintained by ECMA today. Many large telecommunications companies built large proprietary and expensive PBX solutions on top of the CSTA open standard.
Then in 1999, Mark Spencer of Digium, Inc created Asterisk, an open source implementation of a PBX. By supporting a mix of traditional and VoIP telephony services, Asterisk allows developers to build new telephone systems, or gradually migrate existing systems to new technologies. Some sites are using Asterisk servers to replace proprietary PBXs; others to provide additional features (such as voice mail or phone menus) and others to cut costs by carrying long-distance calls over the Internet. Since Spencer released Asterisk to the world in 1999 as an open source telephone operating system, it has been downloaded 500,000 times, and is currently downloaded 1,000 times per day. Some 350 open source contributors have taken it from a basic voice system to one with clear calling and more than 100 features. Asterisk is now released under GPL (GNU General Public License). The benefits of open source initiatives are well understood. It is a very pragmatic way of evolving software in a rapidly changing environment. It harnesses the collective wisdom, experiences, expertise and requirements of its most demanding users to ensure that their needs are rapidly met." (http://openlynx.wiki.sourceforge.net/Open+Source+for+Open+Systems+an+overview)