Asterisk - An open source telephony switching and private branch exchange service for Linux.
Element.io - Secure messaging app with strong end-to-end encryption, advanced group chat privacy settings, secure video calls for teams, encrypted communication using Matrix open network. Riot.im is now Element.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
ChatSecure - ChatSecure is a free and open source messaging app that features OTR encryption over XMPP.
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.
Otr.to - A secure peer-to-peer chat in your web browser.