Asterisk!

Wazo is mainly an IPBX that provides communication services such audio, video, and instant messaging based on the use of standard network technologies such as the TCP/IP protocol. It is interoperable with most market telephony systems and gives users access to flexible and advanced services without requiring specialized hardware.

3CXPhone is a free VoIP phone available for Windows, iPhone or any Android based smart phone such as Google Nexus, Sony Xperia, Motorola Droid or Samsung Galaxy. Connect 3CXPhone to a VoIP provider or to a VoIP PBX (for example 3CX Phone System) to make calls to any VoIP, mobile or landline number.

FreeSWITCH is a scalable open source cross-platform telephony platform designed to route and interconnect popular communication protocols using audio, video, text or any other form of media. It was created in 2006 to fill the void left by proprietary commercial solutions. FreeSWITCH also provides a stable telephony platform on which many telephony applications can be developed using a wide range of free tools.

Zent.IO is a Contact Center Cloud Software Multichannel that unify chat, phone and e-mail. All the communication channels will be answered from a single interface. The agent will only need a web browser

XiVO is a scalable platform based on Asterisk that offers a free and complete PBX solution. It is licensed under the GPLv3.

RingCentral delivers high quality, reliability and value that's expected by Enterprise businesses through RingCentral Office, an all-inclusive cloud phone system containing HD video meetings, unified voice, fax, text, and audio conferencing.

FusionPBX is an open source FreeSWITCH GUI. FusionPBX can be used as a high available single or domain based multi-tenant PBX, carrier grade switch, call center server, fax server, voip server, voicemail server, conference server, voice application server, appliance framework and more. FreeSWITCHtm is a highly scalable, multi-threaded, multi-platform communication platform. FusionPBX license is the MPL 1.1 license.

Unchained resources : Voximal integrates cloud speech services like Google Speech API, IBM Watson SpeechToText, Azure Bing Speech, but keeps on-premise capability thanks to UNIMRCP connector.

Yate is a softswitch (concept of separating the network hardware from network software).

Starfish PBX aims to be an open source alternative to Digium’s Switchvox. The goal is to have a software equivalent to which we can add additional functionality like:

Kamailio® (successor of former OpenSER and SER) is an Open Source SIP Server released under GPL, able to handle thousands of call setups per second. Among features: asynchronous TCP, UDP and SCTP, secure communication via TLS for VoIP (voice, video); WebSocket support for WebRTC; IPv4 and IPv6; SIMPLE instant messaging and presence with embedded XCAP server and MSRP relay; asynchronous operations; IMS extensions; ENUM; DID and least cost routing; load balancing; routing fail-over; accounting, authentication and authorization; support for many backend systems such as MySQL, Postgres, Oracle, Radius, LDAP, Redis, Cassandra, MongoDB, Memcached; XMLRPC control interface, SNMP monitoring. It can be used to build large VoIP servicing platforms or to scale up SIP-to-PSTN gateways, PBX systems or media servers like Asterisk™, FreeSWITCH™ or SEMS. Kamailio and the SIP Express Router (SER) teamed up for the integration of the two applications and new development. »

VoipSwitch is a SoftSwitch System with routing and billing, with SIP protocol support (in older versions also H323).

Flexisip is a SIP proxy server implementation compliant to RFC3261, written in C++ 11.

A SIP based implementation of a PBX / unified communications system.

Thirdlane Business PBX platform provides all of the easy-to-use advanced communications features businesses need, with the flexibility for customization and growth - at a fraction of the cost of competitive solutions.