Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs
Asterisk - An open source telephony switching and private branch exchange service for Linux.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
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.
Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.