I took a quick listen to the audio, and "Bell 202" immediately came to my mind. It crops up carrying Caller ID on landlines in North America, and railway end-of

I took a quick listen to the audio, and "Bell 202" immediately came to my mind. It crops up carrying Caller ID on landlines in North America, and railway end-of-train devices use it for sending their telemetry over narrowband FM. It's very well-suited for any situation where you want to send short, low-bandwidth bursts of data over an audio channel. If you need to encode or decode Bell 202 or similar schemes such as V.23, the SpanDSP library [1] is very handy. [1] https://github.com/jart/spandsp https://github.com/jart/spandsp