The Asterisk software includes many features available in commercial and proprietary PBX systems: voice mail, conference calling, interactive voice response, and automatic call distribution. Users can create new functionality by writing dial plan scripts in several of Asterisk's own extensions languages, by adding custom loadable modules written in PHP or C, or by implementing Asterisk Gateway Interface programs using any programming language capable of communicating via the standard streams system or by network TCP sockets. Asterisk supports several standard voice over IP protocols, including the Session Initiation Protocol, the Media Gateway Control Protocol, and H.323. Asterisk supports most SIP telephones, acting both as registrar and back-to-back user agent. It can serve as a gateway between IP phones and the PSTN via T- or E-carrier interfaces or analog FXO cards. The Inter-Asterisk eXchange protocol, RFC 5456, native to Asterisk, provides efficient trunking of calls between Asterisk PBX systems, in addition to distributing some configuration logic. Many VoIPservice providers support it for call completion into the PSTN, often because they themselves have deployed Asterisk or offer it as a hosted application. Some telephones also support the IAX protocol. By supporting a variety of traditional and VoIP telephony services, Asterisk allows deployers to build telephone systems, or migrate existing systems to new technologies. Some sites are using Asterisk to replace proprietary PBXes, others provide additional features, such as voice mail or voice response menus, or virtual call shops, or to reduce cost by carrying both local and long-distance calls over the Internet. In addition to VoIP protocols, Asterisk supports traditional circuit-switching protocols such as ISDN and SS7. This requires appropriate hardware interface cards, marketed by third-party vendors. Each protocol requires the installation of software modules. In Asterisk release 14 the Opus audio codec is supported.
Internationalization
While initially developed in the United States, Asterisk has become a popular VoIP PBX worldwide. It allows having multiple sets of voice prompts identified by language as well as support for time formats in different languages. Several sets of prompts for the interactive voice response and voice mail features are included with Asterisk: American, British, and Australian English, Canadian French, Japanese, Russian, Mexican Spanish and Swedish. A few novelty prompts are offered, such as jokes and a themed "zombie apocalypse" message for Halloween. Additionally, voice sets are offered for commercial sale in various languages, dialects, and genders. The default set of English-language Asterisk prompts are recorded by professional telephone voice Allison Smith.
Derived products
Asterisk is a core component in many commercial products and open-source projects. Some of the commercial products are hardware and software bundles, for which the manufacturer supports and releases the software with an open-source distribution model.
AskoziaPBX, a fork of the m0n0wall project, uses Asterisk PBX software to realize all telephony functions.
AstLinux is a "Network Appliance for Communications" open-source software distribution.
Issabel is an open-source Unified Communications software which uses Asterisk for telephony functions. It was forked from the open-source versions of Elastix when 3CX acquired it.
*astTECS uses Asterisk in its VoIP and mobile gateways.
Various add-on products, often commercial, are available that extend Asterisk features and capabilities. The standard voice prompts included with the system are free. A business can purchase matching voice announcements of its company name, IVR menu options and employee or department names at additional cost for seamless integration into the system. Other add-ons provide fax support, text-to-speech, additional codecs and new features. Some third-party add-ons are free; a few even support embedded platforms such as the Raspberry Pi.
