Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems

Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.

XO Hosted PBX - Hosted PBX is a scalable UC solution that uses cloud technology to integrate HD voice & data services, mobility, along with advanced calling

StealthChat - StealthChat is Encrypted, Private, Secure Calls and Chats on Android and Apple iOS

Sipdroid - Free and open-source SIP/VoIP Client

Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.