We trust on open source and standars
Asterisk is the leading Open Source software implementation of a telephone private branch exchange (PBX); it allows attached telephones to make calls to one another, and to connect to other telephone services, such as the public switched telephone network (PSTN) and Voice over Internet Protocol (VoIP) services. It can be software only, moving calls around via IP, or it can support a large variety of relatively inexpensive hardware interfaces to directly tie in with existing TDM (Time Division Multiplexing) equipment.
The Media Resource Control Protocol (MRCP) allows applications to control media service speech resources residing in servers. The MRCPv2, is currently a proposed standard protocol. UniMRCP is an open source cross-platform implementation of the MRCP client and server in the C/C++ language distributed under the terms of the Apache License 2.0. The implementation encapsulates SIP, RTSP, SDP, MRCPv2, RTP/RTCP stacks and provides integrators with an MRCP version consistent API.
The FreePBX EcoSystem is the most widely deployed Open Source PBX platform in use across the world today. The openness of the project allows to build robust communications solutions that are powerful but at the same time easy to implement and support. As web-based PBX solution, FreePBX is easy to customize and adapt to your changing needs. It can run in the cloud or on-site, and is currently being used to manage the business communications of all sizes and types of businesses.
Elastix is an unified communications server software that brings together IP PBX, email, IM, faxing and collaboration functionality. It has a Web interface and includes capabilities such as a call center software with predictive dialing. The Elastix functionality is based on Open Source projects including Asterisk, FreePBX, HylaFAX, Openfire and Postfix. Those packages offer the PBX, fax, instant messaging and email functions, respectively. Elastix is Free software, released under the GNU General Public License