Before modern digital electronics, telephone numbers were literal routes - when the turned dial on your phone ran back to zero, a corresponding 10-pole motorised rotary switch at the exchange turned and connected you to one of 10 lines. This connected you to another such rotary switch for the next digit, until eventually you were connected to the final destination. The ingenious Strowger exchange.
Also, every phone had its own physical circuit to the exchange, leading to things like this: https://i.redd.it/ugvoc90k4q5a1.jpg
Invented by a paranoid undertaker out of business interest, apparently:
"Strowger, an undertaker, was motivated to invent an automatic telephone exchange after becoming convinced that the manual telephone exchange operators were deliberately interfering with his calls, leading to loss of business."
I wonder if the phone company was actually out to get him!
And when there was a bug in that complex and vast routing system somewhere, it was completely unfixable. Not without million-dollar hardware replacements at least.
It's really surprising to me how little uptake 2600 ultimately ended up having.