No phone number required for DeltaChat.

It packetises and encrypts chats, using email(SMTP) as the transport medium. Can send downsampled pics, videos or Push-to-talk audio. Can send full quality pics, videos, or attachments too.

It's on F-Droid, and you can use a seperate free email address(100MB limit) for the SMTP backend (from https://nine.testrun.org/ ), or use your own existing email address.

Elegant and robust.