Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
StealthChat - StealthChat is Encrypted, Private, Secure Calls and Chats on Android and Apple iOS
PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
tSIP - tomeko.net/software: SIP client for Windows.