Asterisk - An open source telephony switching and private branch exchange service for Linux.
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Wazo.io - The UCaaS Platform Built for MSPs
8x8 X Series - X Series connects teams across channels and around the globe, giving you access your most critical information securely, from any location.
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.
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.