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?
"Hello how can I help you? - Hi, beep, I like to reserve a table? - Ok, beep, beep, on second - Mhm-mm beep, beep, sceech, 011000101010...."
(Archive link because that blog now requires authorization to view for some reason.)
Good times !