CallHub - Predictive Call Center, Voice and Text Broadcasting app
Asterisk - An open source telephony switching and private branch exchange service for Linux.
CallHippo - Easy to use Phone System with World-Class Support
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.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.