The telephone set is actually a 4 wire device (look at your handset) 2 wires for transmit and 2 for receive. To save cable in the Outside Plant there is a hybrid circuit in the phone to convert 4 wire to 2 wire.

Four wire provides better transmission and reception. Like Ed said, standard T-1s are 4 wire. Tie lines (connections between two switches) used to come in two flavors in the Analog days - 2 wire and 4 wire (E&M).

T-3s, and all fiber connections, while only 2 wire/strand are still transmit & receive and NOT Tip & Ring.

The old US Military Autovon system used 4 wire telephone sets for better (and more secure) communications.

To sum up, 4 wire is better then 2 wire, but it's more expensive and as a result is used much less often.

Sam

"Where are we going and why are we in this hand basket?"