Back in the early 1960s my mother and her friend ran a little diner in north London, with the telephone number LABurnham 1122. She said they frequently took calls intended for a rather large company which had the number LADbroke 1122. It just doesn't pay to have some numbers!
One of the few firsts we Brits can claim over America was the adoption of our 999 emergency number, which started in the 1930s and was in fairly widespread use in dial areas by the 1950s. Under the U.K. system at that time, 999 calls were handled by normal GPO operators -- In fact in most places they were the same operators who took regular "0" (later "100") assistance calls.
On the standard cord boards of the day, the emergency trunks had red call lights over each jack in place of the usual white ones, and every incoming 999 call also operated a klaxon and a large red light atop the boards or on the wall until the trunk was answered. If was then entirely up to the operator to complete the call to the appropriate police, fire, or ambulance department. In many cases, she had dedicated jacks with direct outgoing trunks to each of the major emergency stations for the area to allow the call to be completed as quickly as possible.
Despite the 999 system, there was, however, still a widely adopted convention that the regular number for the police should use 2222. The legacy of this can still be seen in many local police numbers today, e.g. the general (non-emergency) number for my area is 402222. (These days a local number is often routed to a police HQ in a distant town, but that's another story).
The convention also spilled over to companies with large PBX systems, where they had a security officer (or some other person in charge of any emergency situation on the premises) and extension 222 was assigned for emergencies.
Of course, the most famous British police number (excluding 999) didn't follow this convention. For many years the general number for Scotland Yard, Metropolitan Police HQ in London, was WHItehall 1212.
-Paul