Messenger Comparison – Threema, Signal, Telegram, WhatsApp
threema.ch
threema.ch
Except that ProtonMail requires a valid phone number for an account registration (when accessed via Tor).
https://www.reddit.com/r/ProtonMail/comments/638ykr/phone_nu...
Also dedicate a browser to the web client and it's like having iMessage on Linux, Mac and Windows with notifications.
They also have an enterprise edition which is more expensive.