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.

To send a simple text as "Hello" it takes 18 seconds which is excessively long.

There is a library called rattlegram which is well tested and would transmit the same data in less than half a second: https://github.com/aicodix/rattlegram

I'm developing with more people a platform called geogram, which basically looks into transmitting data with radios, bluetooth, wifi, USB-C cables or anything you have. We don't have modem over the phone yet. Maybe I can invite you to join so we can solve those obstacles together?

The website is https://geogram.radio