Asterisk - An open source telephony switching and private branch exchange service for Linux.
CorvisaOne - CorvisaOne is another cloud-based communications suite that is specially designed for companies that need features for managing inbound or outbound call centres.
Wazo.io - The UCaaS Platform Built for MSPs
ShoreTel - Cloud phone system & contact center software & services
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.
8x8 X Series - X Series connects teams across channels and around the globe, giving you access your most critical information securely, from any location.