The Essential Guide to Open-Source VoIP

Updated: April 30, 2009

With much of the technology world barreling headlong toward open-source solutions , it's no surprise that VoIP technology is moving rapidly in the same direction.

While leading hosted VoIP services such as Vonage Holdings Corp ., Aptela Inc . or Vocalocity are strictly for-profit operations, a growing number of people and businesses are moving to open-source VoIP systems and tools that promise not only reduced costs but added flexibility, access to innovative features, improved user support and an array of other advantages. Efforts such as Digium Inc.'s Asterisk , Fonality's Trixbox , Pingtel Corp.'s SIPfoundry , the Open Source Initiative, Free Software Foundation Inc.'s GNU Project and SourceForge Inc. are working to bring an array of VoIP-oriented open-source projects to fruition, as well as to support ongoing development efforts.


Open-source VoIP adopters are looking for the same benefits as most other open-source enthusiasts, including low or no capital costs, community-driven support, easy adaptability, no vendor commitments, a wide array of free or low-cost add-ons and enhanced security. On the other hand, open-source users must be willing to deal with a lack of vendor-sponsored support (including emergency and on-site service), software that may require more technical expertise than its commercial counterparts and a less-polished appearance to user interfaces and other controls. However, many open-source VoIP technologies are supported by commercial businesses that give their customers a training and support foundation without tying them into long-term product commitments.


Open-source VoIP development has now reached the stage that there is an open-source equivalent for just about every commercial VoIP solution. Here's a rundown of the top offerings:

PBX and IVR Platforms

Asterisk: It might as well be called THE open-source communications platform. Asterisk is designed to serve as a complete IP PBX in software. The offering runs on a wide variety of operating systems, including Linux. It is the dominant open-source PBX solution on which many others are based.

SIPfoundry: Less well-known than Asterisk, SIPfoundry is perhaps the best open-source alternative. It is 100 percent SIP (Session Initiation Protocol) standards-based, unlike Asterisk, and it uses a very easily configurable and extensible stack that allows companies like Pingtel to build enterprise-class PBXes on top of the open-source stack.

Trixbox: Trixbox is an Asterisk-like PBX based on the Asterisk core. It started to diverge from Asterisk several years ago, however, and now offers alternative components at various levels in the software stack.

GNU Bayonne: This is a free, scalable telecommunications application server from the GNU Project.

CallWeaver: Formerly known as OpenPBX, CallWeaver is another Asterisk branching. It is less commercially driven than Asterisk itself or Trixbox, both of which have major vendors behind them.

SIP Proxies

OpenSer: OpenSer is a flexible, open-source SIP server. It can be used on systems with limited resources as well as on carrier-grade servers, scaling to up to thousands of call setups per second.

PartySIP: This is a SIP proxy server. Besides standard routing rules, PartySIP can get extra routes from additional plug-ins.

SIP Express Router: A high-performance, configurable SIP server licensed under the open-source GNU license. It can act as SIP registrar, proxy or redirect server.

SipX: SipX is an open-source VoIP telephony server.

SIP Clients

Linphone: Linphone is an open-source SIP videophone for Linux and Windows systems.

Phonegaim: It's a free software VoIP system based on the Pidgin instant-messaging software and the SIP protocol.

WengoPhone: A SIP client with voice, video, SMS, chat and presence capabilities, WengoPhone is produced by OpenWengo under an OSI-certified license.

Cockatoo: Cockatoo is software that allows users to place a phone call by clicking a line in an address book, as well as to reply to emails by phone.

Minisip: This is a SIP phone that can be used to instant message and make phone calls and video calls to individuals connected to the same SIP network.

Shtoom: Shtoom is an open-source, cross-platform VoIP softphone.

Twinkle: Twinkly is a VoIP softphone that uses the SIP protocol.

H.323 Clients

YATE: This is a telephony software engine that can be used as a client or a server. The offering includes support for VoIP protocols and hardware interfaces. It also supports IVR (interactive voice response), vocal prompts and a variety of other features.

FreeSWITCH: FreeSWITCH is an open-source telephony platform that's designed to enable the creation of voice- and chat-driven products scaling from a softphone up to a soft-switch. It can be used as a simple switching engine, a media gateway or a media server to host IVR applications using simple scripts or XML to control the call flow.

XMeeting: It's the first H.323-compatible video-conferencing client for Mac OS X.

IAX Clients

IAXComm: This is an open-source softphone for the Asterisk IP PBX.

Kiax: An IAX (Inter-Asterisk eXchange) client application that allows PC users to make ordinary VoIP calls to Asterisk servers.

YakaPhone: YakaPhone is an IAX-based softphone.

SFLPhone: It aims to serve as an enterprise-class desktop phone, targeted at people who handle a large number of calls, such as receptionists.

Stacks and Libraries

OpenSIPStack: This is an ongoing effort to offer software developers a platform-agnostic stack implementation of the RFC 3261 definition.

The GNU oSIP Library: It aims to give multimedia and telecom software developers an easy and powerful interface to initiate and control SIP-based sessions in their applications.

Future Outlook

Open-source VoIP development shows no signs of slowing down. In fact, the field of available technologies is likely to continue growing, with new technologies arriving and older offerings being updated on a continual basis. One place to keep an eye on new and cutting-edge VoIP technologies is theeComm Conference 2008.

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