Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
StealthChat - StealthChat is Encrypted, Private, Secure Calls and Chats on Android and Apple iOS
Asterisk - An open source telephony switching and private branch exchange service for Linux.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
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.