DeStar is a web based high-level management and configuration tool for the open-source PBX Asterisk

FreeSwitch is a telephony application built from the ground up and designed to take advantage of as many existing software libraries as possible. FreeSWITCH is an open source telephony platform designed to facilitate the creation of voice and chat driven products scaling from a soft-phone 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 callflow. It supports various commu

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OpenPBX.org is a community driven software PBX project.

The popular and extensible open source telephone system in the world, offering flexibility, functionality and features not available in advanced, high-end (high-cost) proprietary business systems. Asterisk® is a hybrid TDM and packet VOIP voice PBX (Private Branch eXchange) and IVR platform with ACD functionality. Derivatives rPath Asterisk software appliance TrixBox

VoIP or Voice over IP is a term used to describe the technique of transmitting real time (usually voice) communications over IP networks. This is in contrast to traditional voice communications, which runs over a network of lines using time division multiplexing. VoIP systems can reach “normal” phones (wired and mobile) through gateways connecting the two networks. This technique is used to supply multiple types of products and services: PBX systems, where VoIP-enabled equipment creates

Trixbox is a pbx linux distribution based on CentOS that aims to make setting up a VoIP systems very quick and easy, using the Asterisk project.