Asterisk - An open source telephony switching and private branch exchange service for Linux.
Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
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.
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
Wazo IP Telephony - Wazo IP Telephony is an open-source platform that is designed to build your own IP telecom platform and provides IP telecopy, mobility.
JivoChat - JivoChat is a live chat that enables you to talk to visitors on your website