threelonmusketeers

https://signal.org/

https://en.wikipedia.org/wiki/Signal_(software)

Signal is an open-source, encrypted messaging service for instant messaging, voice calls, and video calls

It is developed by the nonprofit Signal Foundation and its subsidiary Signal Messenger LLC. Signal’s software is free and open-source. Its mobile clients, desktop client, and server are all published under the AGPL-3.0-only license

I finally got my parents onto it, and have been pretty happy with it as an alternative to Facebook Messenger.