I did the same thing to a numpad phone (also old) about 2 months ago. I squeezed a pi zero-w into the chassis and wrote some software to bind each key from 0-9

I did the same thing to a numpad phone (also old) about 2 months ago. I squeezed a pi zero-w into the chassis and wrote some software to bind each key from 0-9 to a song and then used the Pi's Bluetooth module to play the sound on a speaker across the room. I also have a button that reads out the current time and it runs a website to upload new songs and change the bindings. It was a really fun project and I am surprised someone had the same idea!