[Animats]

The original 911 system was rolled out when many phone switches were electromechanical. Step-by-step and crossbar offices didn't log calls, didn't pass line identification data with the call, and couldn't route based on the source of the call. But they had hard-wired relay logic in the originating registers (the units that receive dial digits) to recognize area codes (second digit 1 or 0 back then) and service numbers (second and third digits 1). Those were forwarded to a toll office, along with automatic line identification info. So 911 was forwarded to a toll office, like a long distance call. "999" was considered as the emergency number; the UK used that. But US switches didn't have a special case for 999, so that was hard to retrofit. Hence 911.

Toll offices, which handled long distance calls, were computerized by then. They could be upgraded to deal with 911 routing. This requires a database to map phone numbers to the appropriate Public Safety Answering Point. The idea of having a computer with a disk drive in the call chain was radical at the time, but it was made to work. The original computers for this were Western Electric 3B machines, set up as duplex pairs.

The toll office 911 system became a problem with the breakup of AT&T. AT&T retained most of the toll switches, while the local exchange carriers got the local offices. (Now, of course, with weak antitrust enforcement, they've all merged back together again.)