Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.

Asterisk - An open source telephony switching and private branch exchange service for Linux.

tawk.to - tawk.to is a free live chat app that lets you monitor and chat with visitors on your website or from a free customizable page

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.

JivoChat - JivoChat is a live chat that enables you to talk to visitors on your website

FusionPBX - FusionPBX is an open source FreeSWITCH GUI.