[henrieri]

If both sides were to use Duplex, it would already know to just send 1010 instead of verbal communications.

Also if it was unknown whether the opposing person was a bot, a bot could firstly send a common test according to some protocol to ask if the other one was bot by some kind of sound representing that. In which case both would start sending machine readable information to each-other.

[extralego]

Sounds a lot like a dialup handshake.

[snvzz]

Or just hang up automatically.