This[1] is something I've come across but not had a chance to play with, designed for reading non-smart meters that might work for you. I'm not sure if there's any way to run it on an old phone though.
Wow. I was looking at hooking my water meter into home assistant, and was going to investigate just counting an optical pulse (it has a white portion on the gear that is in a certain spot every .1 gal) This is like the same meter I use, and perfect.
(It turns out my electric meter, though analog, blasts out it's reading on RF every 10 seconds unencrypted. I got that via my RTL-SDR reciever :) )
[1] https://github.com/jomjol/AI-on-the-edge-device