Linphone - open source SIP client

SIP (VOIP) phoneGPL-3.0-only

Sipdroid

A SIP (VOIP) clientGPL-3.0-only

baresip

VoIP and mobile calling and messaging app for Android based on baresip libraryBSD-3-Clause

baresip+

VoIP and mobile calling and messaging app for Android based on baresip libraryBSD-3-Clause

Jami

Share, freely and privatelyGPL-3.0-or-later

Nextcloud Talk

Have private video calls and chat using your own server.GPL-3.0-only