Ackshully it's from the even older phones with separate earpiece and fixed microphone. You'd really hang up the earpiece on these old ones.

You'd physically hang up wall phones, even when they had a combined microphone and speaker: https://images-na.ssl-images-amazon.com/images/I/515Mj67cq3L...

Yes, but what I meant was that the expression is likely even older than from the ones which went out of style 30 years ago.