Oh.
I guess you never dealt with one of these. Each wire pair = one (analog at this point) landline telephone (one "channel" on the machine).
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The channels of the switch are cabled (hidden) to those blocks. Channel 1 through how ever many thousands, in chunks of 10,000 for large offices - last four digits of a phone number 0000-9999).
Another set of similar blocks are connected to the copper that goes outside and down the road to the houses and businesses. They look a little different, usually vertically oriented, and have lightning arrestors on each pair. Here, they used red and white jumoers.
The jumper wires above connect a specific channel on the machine to a specific pair on the outside. There is no rhyme or reason to the pattern, for a new subscriber, an unused channel is jumpered to the cable pair that goes to his phone, and remains until a fault, or the subscriber stops paying the bill, and someone else is assigned that now vacant machine channel.
It's all unshielded, but is twisted, to avoid (most) instances of crosstalk.
I didn't mind doing small offices - a couple of thousand lines - places like Iola Wisconsin, and Show Low Arizona. Fortunately my exposure to the big ones was much more limited - Cincinnatti Bell had five big ones, I visited to help out for a week. Remember watching the Berlin Wall come down at the motel.