Analogue, or for that matter digital audio, is often carried over two copper wires twisted together called a balanced-line or twisted pair. Where one twisted pair is used to send audio, with a separate pair used to receive, communications jargon calls this a “four wire”. A very simple analogue talkback station or a microphone-headset might use a four wire
It’s possible to use this single balanced line, to transport carry audio in both directions, send and receive on the same line. This is called a “two-wire”.
Talkback systems, even those which are digitally implemented, need to be able to interface to two-wire and four wire analogue connections at their edge.