And the secret knowledge that a specific whistle from a cereal box - blown into the phone at just the right time - can land you a Saturday night reservation at French Laundry.

they could just encode the message in a way that’s inaudible to humans.

So a simple “hello” contains a subliminal audio signature that basically is code for “I am a robot. Are you a robot? Feel free to beep bop”

Machine A extends the "hellooo" out to exactly 1.32 seconds, with a 1.32 second pause between that and the next word.

Machine B responds with a greeting exactly 1.32 seconds long, with a 1.32 second pause after it.

Machine A responds with a sentence in which the first word is exactly 1.32 seconds long.

Machine B considers the handshake a success and proceeds in machine language.

Oh dear, polyglot to audio steganographic phone calls, the audible message doesnt even has to be the same then the one encoded in microgaps and envelope shifts and all kind of modulations.

That's right- I started thinking about it because of this: https://news.ycombinator.com/item?id=16046329

Surely someone out there is doing audio steganography by modulating the volume/duration of "silence".