I did a similar thing a couple of years ago, but with a TV instead of a phone (and for my dad).

https://blog.davidv.dev/revamping-an-old-tv-as-a-gift.html

The gist of it was putting a Pi inside a b/w CRT, soldering the GPIO to the tuner and that way you could change the channels. I even put fake static on some of them.

The software ended up horrible as I ran out of time.. but it worked great!