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

freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).

HelpScout - Help Scout is a simple, straightforward way to provide excellent support