StealthChat is an industrial strength chat and phone call app for professionals that value real privacy and security. StealthChat uses advanced end to end encryption to ensure that only the sender and recipient have access to the content of the message or phone call.
Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).
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.
tSIP - tomeko.net/software: SIP client for Windows.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.