15.2.2 (21 February 2018; 44 days ago (2018-02-21)[1]) [±]

13.19.2 (21 February 2018; 44 days ago (2018-02-21)[2]) [±] Preview release

15.3.0-rc2 (March 8, 2018; 29 days ago (2018-03-08)) [±]

13.20.0-rc2 (March 8, 2018; 29 days ago (2018-03-08)) [±] Written in C Type Voice over Internet Protocol License GPLv2 with additional licenses available from Digium[3] Website asterisk.org

Asterisk is a software implementation of a telephone private branch exchange (PBX). It allows telephones interfaced with a variety of hardware technologies to make calls to one another, and to connect to telephony services, such as the public switched telephone network (PSTN) and voice over Internet Protocol (VoIP) services. Its name comes from the asterisk symbol "*".

Asterisk was created in 1999 by Mark Spencer of Digium.[4][5] Originally designed for Linux,[6] Asterisk runs on a variety of operating systems, including NetBSD, OpenBSD, FreeBSD, macOS, and Solaris, and can be installed in embedded systems based on OpenWrt and on flash drives.[7][8]


The Asterisk software includes many features available in commercial and proprietary PBX systems: voice mail, conference calling, interactive voice response (phone menus), 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 C, or by implementing Asterisk Gateway Interface (AGI) programs using any programming language capable of communicating via the standard streams system (stdin and stdout) or by network TCP sockets.

Asterisk supports several standard voice over IP protocols, including the Session Initiation Protocol (SIP), the Media Gateway Control Protocol (MGCP), 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 public switched telephone network (PSTN) via T- or E-carrier interfaces or analog FXO cards. The Inter-Asterisk eXchange (IAX) protocol, RFC 5456, native to Asterisk, provides efficient trunking of calls between Asterisk PBX systems, in addition to distributing some configuration logic. Many VoIP service 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 long-distance calls over the Internet (toll bypass).

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.


While initially developed in the United States, Asterisk has become a popular VoIP PBX worldwide because it is freely available under open-source licensing, and has a modular, extensible design. It allows having multiple sets of voice prompts identified by language (and even multiple sets of prompts for each 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.[9] A few novelty prompts are offered, such as jokes[10] and a themed "zombie apocalypse" message for Halloween.[11] 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.[12]

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.
  • Elastix uses Asterisk, HylaFAX, Openfire and Postfix to offer PBX, fax, instant messaging and email functions, respectively.
  • FreePBX, an open-source graphical user interface, bundles Asterisk as the core of its FreePBX Distro[13]
  • LinuxMCE bundles Asterisk to provide telephony; there is also an embedded version of Asterisk for OpenWrt routers.
  • PBX in a Flash/Incredible PBX and trixbox are software PBXes based on Asterisk.

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 (as a library of live recordings of common names[14] or a set of fully customised prompts recorded by the same professional voice talent) at additional cost for seamless integration into the system.

Other add-ons provide fax support, text-to-speech, additional codecs and new features.[15] Some third-party add-ons are free;[16] a few even support embedded platforms such as the Raspberry Pi.[17]

See also


  1. ^ "ChangeLog-15-current". Retrieved 21 February 2018. 
  2. ^ "ChangeLog-13-current". Retrieved 21 February 2018. 
  3. ^ "Asterisk LICENSE". 
  4. ^ Olejniczak, Stephen P.; Kirby, Brady (2007). Asterisk For Dummies. John Wiley & Sons. ISBN 9780470098547. 
  5. ^ Van Meggelen, Jim; Smith, Jared; Madsen, Leif (2007). Asterisk: The Future of Telephony. O'Reilly Media, Inc. ISBN 9780596510480. 
  6. ^ The README for version 0.1.0 states: "Currently, the Asterisk Open Source PBX is only known to run on the Linux OS, although it may be portable to other UNIX-like operating systems as well." See here
  7. ^ "Asterisk on OpenWrt". Retrieved 2011-10-09. 
  8. ^ AstLinux: Boot via USB Flash Storage
  9. ^ download page of sound files for Asterisk
  10. ^ ץ "You are not the next caller in line", parody on-hold message where a pre-recorded Allison Smith sheepishly confesses (at 0:00:45) that the caller is actually *not* next in queue and would be lucky to get a response at 11:30pm from the cleaning lady after other workers had left for the day.
  11. ^ "Zombie-Proof Your Phone System". Go.digium.com. Archived from the original on 2016-03-05. Retrieved 2016-01-04. 
  12. ^ "Asterisk Voice Recordings". The IVR Voice. 
  13. ^ Madsen, Leif; Jim Van Meggelen; Russell Bryant (2013). Asterisk: The Definitive Guide, 4th Edition (4th ed.). O'Reilly Media. p. 800. ISBN 978-1-4493-3242-6. FreePBX, the juggernaut of the Asterisk community. This interface (which is at the heart of many of the most popular Asterisk distributions, such as AsteriskNOW, Elastix, the FreePBX Distro, and PBX in a Flash), is unarguably a very large part of why Asterisk has been as successful as it has. With the FreePBX interface, you can configure and manage many aspects of an Asterisk system without touching a single configuration file. While we purists may like everyone to work only with the config files, we recognize that for many, learning Linux and editing these files by hand is simply not going to happen. For those folks, there is FreePBX, and it has our respect for the important contributions it has made to the success of Asterisk. 
  14. ^ "Allison On Demand". AsteriskExchange.com. 
  15. ^ "Asterisk Software Add-Ons". Digium. 2015-12-29. Retrieved 2016-01-04. 
  16. ^ Riddell, Matt (2009-08-08). "35 Great free Asterisk applications". Venturevoip.com. Retrieved 2016-01-04. 
  17. ^ "The 5-Minute PBX: Incredible PBX 11 and Incredible Fax Get a Facelift". Nerd Vittles. 2013-06-18. Retrieved 2016-01-04. 

External links