http://e55lzoju6enqswtvm66sk3uqukcin3377ehgiorplj4d7qyssipls4yd.onion/en/blog/what-messenger-apps-should-i-use-to-maintain-full-anonymity.php
It runs on Windows, Linux, Android, iOS, and Mac. As a user, you are prompted to connect your mobile number to the app to make it easier for their system to run on multiple devices, such as when you have to be on your computer but also need to respond to calls.