It’s fascinating to realize that the work we do every day with web servers, which seems like a recent modern technology, was predated by guys like Erlang working with early phone switches over 100 years ago. Phone switches were the first servers: central machines connected to a large number of potential clients. In building these switches, the early engineers had to figure out from scratch how to anticipate the possible work load, so they could build switches large enough but not too large. The whole of queueing theory springs from the theories worked out by telephone switch engineers.
And they were clever guys, even adjusting the UI to lighten the load on the switches. When dialing a rotary phone, the particular digits determined how long the switch was engaged before the call could be routed. So when they allocated area codes,
the biggest population areas [got] the numbers that took the shortest time to dial on rotary phones. That is why New York City was given 212, Los Angeles given 213, Chicago 312, and Detroit 313, while Vermont received 802 (a total of 20 clicks, 8+10+2). Four areas received the then-maximum number of 21 clicks: South Dakota (605), North Carolina (704), South Carolina (803), and Nova Scotia/Prince Edward Island in the Canadian Maritimes (902).