VitalPBX is a Unified Communications System that has everything we were looking for, from Call Center modules, call recording, easy to install in the cloud and the most impressive thing is the ease of configuring it and how stable it is.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
3CX Phone System - 3CXPhone is a free VoIP phone available for Windows, iPhone or any Android based smart phone such...
FreeSWITCH - FreeSWITCH is an open-source media application designed to support popular protools such as SIP and WebRTC and provides a platform to develop voice and video applications.
Wazo - Wazo is a unified communications platform based on Asterisk and focused on extensibility.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies