I prepared this short explanation for a friend. As we have some telecoms-interested folk in ECN, I thought it might be of passing interest to post here as well.
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The British 999 emergency system was inaugurated in London on July 8, 1937, and it's on record that the first call to the new service was made at 4:20 a.m. by a householder reporting a burglar (who was later apprehended).
Why the number 999? Why not 111, 222, or some other number? There are several factors which came into play, although I've never seen a full official explanation of the choice. Some of my thoughts on the matter:
First, in the 1930s it was still held by the GPO that no valid numbers should start with a 1. (The problem being that somebody fumbling with the handset as he picked it up would accidentally send a single pulse.) Level 0 in those days was reserved solely for reaching an operator. So that left only 2 through 9 as an initial digit.
As the emergency number was to be used right across the country, it had to be fitted into all existing exchange numbering schemes with the minimum of disruption. Outside of London and the other director areas, just about all exchanges at that time started assigning numbers from level 2, e.g. local numbers may have been 214, 2539, or 20340. When level 2 was full, they proceeded to levels 3, 4, 5, etc. Having a new emergency code starting with any of those digits would have involved major changes. In these smaller exchanges, levels 6, 7, and 8 were commonly used for accessing direct trunks to neighboring exchanges (this was 20 years before long-distance dialing was introduced, but in many areas subscribers could call neighboring local exchanges automatically).
So we come to level 9, which in the 1930s was already in use for many standard service codes, for example, 91 for general inquiries, 92 for directory inquiries, 90 for telegrams etc. Many small rural exchanges were operated as satellites from a central parent exchange. As operators were stationed only at these centers, many small rural offices were already arranged such that dialing an initial 9 dropped the caller into a trunk to the parent exchange. Thus the use of 999 meant that no re-trunking was needed at all those small dependent exchanges.
Another point concerns calls originating from coin phones, or "public call offices" as they were known at that time. GPO coin phones were the pre-payment type, and the dial was inoperative until money had been deposited. Except for dialing 0 that is -- The dials on coin phones had special sets of contacts so that 0 could be dialed to reach an operator without inserting any coins. The GPO considered it important that an emergency call could be made without coins, and it wasn't too difficult to modify the dial mechanism to allow both 9 and 0 to be dialed. Had the emergency number contained ANY other digits besides 9 and 0, the task would have been made harder.
Finally, there's the question of exchange codes that were already in use in London and the other director areas (those cities with 7-digit numbering). British 7-digit numbers were of the form 3L-4N (e.g. the famous number for Scotland Yard, WHItehall 1212). Director exchanges were equipped with register translators which accepted the first three dialied digits and looked up the proper routing code to the required exchange (e.g. if the first three digits were 723 the destination office was PADdington). As the digit 9 corresponds to the letters W,X,Y, there were no exchanges in any of the director areas using the prefix 999. Thus the translators could easily be arranged to route a 999 prefix to the emergency trunks. Some other three-digit codes in director areas were also reserved for "special" use, e.g. 364 (ENG) engineering, 846 (TIM) time, etc. (The latter being the reason why some people in Britain today still talk about calling "Tim," even though the time number has long since changed -- But that's another story!)
One final point is that by choosing numbers close to one end of the dial, it becomes easy to dial in a dark or smoke-filled room. Finding the 9 hole by feel alone is much quicker than trying to count forward or back to some other digit near the middle.
[This message has been edited by pauluk (edited 02-08-2003).]
