ThinkGood has raised a good point about the old coin phone lines.
I've never heard of the 3rd-wire 70V system (phone companies would normally try to minimize wiring where possible), but it may well have been used by one of the former independent companies now in the Verizon area.
Many coin phones used a local ground connection -- They were often ground-start, meaning that to get dial tone the phone grounded one side of the line when you inserted your nickel or dime (or dimes/quarter after inflation!).
I know that for many years the Bell System coin phones used a coin collect/return system based on a + or - 120V DC pulse on one side of the line. I forget which polarity was for which function. I don't know if any of these systems are still in use in some remote part of the country.
In the U.K. our old coin phones were less sophisticated, and callers were instructed to press one of two buttons. Button A was coin collect and through a mechanical linkage dropped the coin(s) into the box and removed a short from the transmitter so that the caller could talk. Button B returned the coins (after getting a busy signal for example) and engaged a timer which opened the line for a few seconds to drop the call.
Ma Bell was the more sophisticated!
