Signal is not on F-droid. There is no Guardian repo on F-droid. The link you posted directs to the Guardian page about Signal, but nu info about Signal being on F-droid

Edit : found the repo on https://community.signalusers.org/t/make-signal-available-in-fdroid/2608/161

Guardian repo: https://guardianproject.info/fdroid

Or an AppImage

I know, most people I know stick to whatssucks and faciesbook messenger.

XMPP and Threema don’t require a phone number. And if you want you can run XMPP on your own server.