Signal is the most scalable encryption tool.

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

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

FreeSWITCH is a free and open source application server for real-time communication, WebRTC, telecommunications, video and Voice over Internet Protocol (VoIP)…

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.

Blink is the best real-time communications client using the SIP protocol.