Asterisk - An open source telephony switching and private branch exchange service for Linux.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
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.
MightyCall - Virtual phone system that lets you run your business from anywhere.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
JustCall - The Cloud Phone System of choice for modern sales and support teams. We help end clueless conversations by adding context to every call.