Distance is the problem. The gauge is small so we can't throw a very strong signal down the wire. So you have repeaters on almost every span of any appreciabl

Distance is the problem. The gauge is small so we can't throw a very strong signal down the wire. So you have repeaters on almost every span of any appreciable length. The very first part of a dialup modem sound? Where it's playing a tone that reverses phase at regular intervals? That tone is actually designed to disable all the repeaters and echo cancelers that are in your switched circuit. Also two parallel phone lines are prone to capacitive coupling. I had a case so bad once that one office could pick up the phone and nearly perfectly couple onto their neighbors line and hear all their conversations. It was a 50/50 which port on the PBX recognized the tones and started the call when either of them picked up to dial out.