Earlier quoted context omitted.
To save CPU/GPU/TPU there should be a high-frequency sound, as in people can’t hear, so the computers talking to each other and switch to a faster way to communicate. If this is included you also have way to detect if you are talking to a bot/duplex.
Doesn't most carriers heavily "compress" the sound, removing all sounds/frequencies that a human can't hear, etc? https://www.youtube.com/watch?v=w2A8q3XIhu0
Or just communicate "hey actually connect to this HTTP/XMPP/whatever address on the internet and we'll continue this from there"
1. Probably a bit slower, I've heard modern VoIP lines don't work well with traditional modems?