RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
Asterisk - An open source telephony switching and private branch exchange service for Linux.
Melrose Labs CloudPBX - Advanced Hosted Cloud PBX for Your Team.
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.
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).