Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
Nextiva - Business VoIP, cloud phone systems trusted by more than 150,000 companies. Powered by the leading cloud PBX VoIP platform, Nextiva is rated the best business VoIP service provider.
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.
3CX - Hosted PBX with CRM integration and mobile apps
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).