For those interested to know more and discuss the protocol, here's my attempt at documenting what worked at the end: https://github.com/shayanb/Nedagram/blob/main/docs/PROTOCOL....

I had to make sure the sound could be transferred on phone calls so it has lower baud rate than let's say dial up modems and many other methods (e.g. minimodem), I'd love to find a way to make it better.

Also got a suggestion to mix this with songs so it's less dodgy to play something like this on the phone. something that I want to look into as well.