When I was in the Army in Germany, I worked at the switchboard. We had a few telephone lines between HQ where I was and other missile battalions. These were from the German telephone system. There weren’t enough telephone pairs so they ordered the transformers used to make phantom circuits and installed them at both ends of the telephone lines. The transformers added a third phantom line to two existing lines. Then another phantom line was added to another two telephone lines. But another phone line was needed, so they put a third phantom line onto the two phantom lines. The voice signals on each of the two phantom lines were very weak and could barely be heard. The phantomed phantom line was too weak to be heard but the powerful ringer signal went through and alerted the switchboard operator on the other end, so we knew the phantomed phantom line was otherwise working. There was just too much loss in the circuit to hear the voice.
There were some missile sites that were dozens of miles away and isolated in the countryside. These were communicating by microwave radio to our HQ, and our switchboard was connected to the microwave radio site nearby. There were often times when I plugged the switchboard into the microwave radio lines and they were howling due to feedback, or sometimes the gain was on the verge of howling and sounded hollow like talking into a long tube or pipe. Sometimes those microwave channels were dead due to equipment failures. I think a lot of the howling and hollow sounds were caused by the changes in temperature and the microwave equipment would gain or lose a few dB and break into howling.
https://www.epanorama.net/newepa/2013/04/15/ethernet-and-telephone-can-share-same-wires/
