Sipdroid

A SIP (VOIP) clientGPL-3.0-only

Linphone - open source SIP client

SIP (VOIP) phoneGPL-3.0-only

VoIP.ms SMS

Popular Android messaging app for VoIP.ms, a Canadian VoIP providerApache-2.0

baresip

VoIP User Agent app for Android based on baresip SIP libraryBSD-3-Clause

baresip+

VoIP User Agent app for Android based on baresip SIP libraryBSD-3-Clause

Element - Secure Messenger

Group messenger - encrypted messaging, group chat and video callsAGPL-3.0-or-later

Simlar - secure calls

Protect your privacy with encrypted phone calls.GPL-2.0-or-later

aTalk

XMPP/Jabber client with encrypted instant messaging and video callsApache-2.0

PilferShush Jammer

Block unwanted use of onboard microphoneApache-2.0