As an end user, picking up the phone to hear a beep is not pleasant. I'm likely as not to immediately hang up, as I've come to associate beeps at the start of calls with scammers.
What about if the caller makes no such sounds and the recipient makes the offer to handshake?
Anyways, this aspect is more amusing to just think about than anything else. That said, I really hope companies who produce these next-gen AI robo-callers actually have the courtesy of identifying themselves as such. I want to know if I am talking to a human or Duplex. Yes, I may hang up, but I feel uncomfortable being fooled into thinking I am talking to a human when I am not.
There's no reason why it can't be encoded as elevator music - you already hear it all the time, they might even throw in a looping "Thank you for calling. Your call is important to us" to keep you from freaking out.
Phone lines are optimized for frequencies humans can hear, though I'm guessing you could get enough bandwidth out of the edges to convince the other side you're a machine without bothering a human too much.
