A few photos of our old AT&T Definity 75 PBX.
This shows one type of complexity that I no longer deal with, the tracing of wires. Every phone was on 1 or 3 pair of wires that had to go directly into the PBX. And every outside line also had to go to the PBX. But you don't connect wires directly to the PBX, they connect to the patchboard which in turn connects the wires to a specific PBX port. This makes it easier to manage since the ports are marked on the patchboards. Unfortunately the markings are not consistent and old wires are not removed. While logical I always found this to be a challenge when I had to move wires around, such as when a location switched between a digital and an analog phone. I won’t miss it.