It'd be trivial to detect if they transmit high-frequency noise over the line, and then fallback to DTMF tones.

That'd be a creepy conversation to listen in on. "Ah yes, I see you are a robot too." Proceeds to transmit data through dial-up-esque noise

Since the restaurant in this situation is clearly using a digital booking system already why would Google (or whatever service inherits this kind of bot call) not just check on the popular booking sites before placing the call?