I'll argue that. One of the first cool things I made with a electronics kit as a boy was a 1 wire intercom.
It started off as the 2 wire intercom example from the book, but I cranked the gain of the audio receiver amp right up, and it still worked with only 1 wire connecting the 2 kits (no earth, both powered from a 9v battery).
So it can definitely be done, all you need is to send the signal down the wire at a few volts and have a high gain audio amp at the receiver end.
A tone will be much easier to detect than a single pulse.