I admit I could be missing something though, as I've not used it heavily.
Sign in to matrix.org with Riot Android / IOS / Web
Like sibling poster, XMPP with OMEMO encryption is an option, but it requires setting up an xmpp server with ejabberd or prosody software, then installing Gajim on your PCs and Conversations (paid via play store, free via F-Droid) for Android / a good iOS Xmpp client that supports OMEMO.
Except for techies, ^ is waaaay too much for people who are just looking to have an alternative to Signal's lock-in. I can barely get people to install the signal app and click open :)
I currently recommend installing Riot IOS / Riot Android and logging in to the free Matrix.org instance, which does not use your phone number or anything.
In the future I hope to recommend whatever P2P Matrix phone app comes out, and you just share your ids with friends via QR code handshakes and boom, you are off to the races with nothing but an internet connection and a friend ID. Metadata is all local to phones, so nothing to see.
In the medium future, I also hope to recommend cwtch.im which is the replacement for richochet, a metadata resistant p2p e2ee messaging app.
In the far future, I hope to have something like a smartphone mobile ad hoc network-based p2p e2ee app where you scan qr codes to handshake and you either communicate over the internet (like p2p matrix) or over mesh network as long as you are within 10 hops of another device (like bridgefy or firechat). Bonus points if one can DIY a passive repeater that works with the mesh network.
It's the least open "open source" model, and once (if) they gain significant market share they can easily close down the app and lock-in the users.
Please use and spread federated alternatives. Donate and contribute.