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).

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

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.

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

Phoner and PhonerLite are softphone applications for Windows operating systems available as freeware.