Twilio - Brings voice and messaging to your web and mobile applications.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
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.
CallHippo - Easy to use Phone System with World-Class Support
Asterisk - An open source telephony switching and private branch exchange service for Linux.