Signal is the most scalable encryption tool.

Linphone is a free voice over IP (VoIP) service and SIP client.

CSipSimple is a Voice over Internet Protocol (VoIP) application for Google Android operating system using the Session Initiation Protocol (SIP).

Jitsi (formerly SIP Communicator) is a free and open source multiplatform voice (VoIP), videoconferencing and instant messaging application.

Ring (SFLphone) is an open-source SIP-compatible softphone and instant messenger for Linux, Microsoft Windows, OS X and Android.

Zoiper is an easy to use sip video softphone, with excellent voice quality and easy to setup.