Sipdroid

A SIP (VOIP) clientGPL-3.0-only

VoIP.ms SMS

Send and read SMS via voip.msApache-2.0

Linphone

SIP (VOIP) phoneGPL-2.0-or-later

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 libraryGPL-2.0-only

Element (previously Riot.im)

All-in-one secure chat app powered by MatrixApache-2.0