Any sort of system where you have an band limited audio channel and need signalling is going to end up using tones. DTMF is a lot more robust, but single freque

Any sort of system where you have an band limited audio channel and need signalling is going to end up using tones. DTMF is a lot more robust, but single frequencies work too. See also https://en.wikipedia.org/wiki/Quindar_tones https://en.wikipedia.org/wiki/Quindar_tones