The telephone network at this time was the responsibility of The General Post Office, and The Postmaster General set up a committee to investigate ways to improve emergency communications. In January 1937 the committee recommended a short telephone code be introduced for emergency operator calls and proposed 999. The committee was adamant that any number must be standard accross all telephone exchanges, be easy to remember, and be able to be dialled from kiosks without inserting money. To make the new number easily compatible with the 'Director Strowger' automatic exchanges used in London and other big cities the new code needed to be 3 digits long as this could be easily interpreted by the existing equipment. It also needed to fit into the numbering system already in use in automatic exchanges.
Numbers beginning with 0 were already used for non emergency access to the operator
1 was generally not used because the pulse dialling system could easily lead to a 1 being transmitted to the exchange by accident, either by uninsulated overhead wires tapping together in the wind, or by fumbling the switchhook on the phone when lifting the handset.
For this reason 111 was avoided.
Numbers beginning with 2 were already used for general phone numbers meaning payphones could not discriminate between emergency calls starting with 2 and a call to an ordinary number - hence money would have to be inserted. Furthermore in London 222 was already the code for Abbey Exchange.
Likewise Numbers beginning with 3,4,5,6,7,8 were already used for general phone numbers meaning payphones could not discriminate between emergency calls starting with 2 and a call to an ordinary number - hence money would have to be inserted.
9 was not in use for any other numbers and so payphones could easily be made to distinguish emergency calls and allow the call to be made without money.
At the time (1937) no other country had a similar system for emergency calls and its introduction on all 91 automatic 'Director Strowger' exchanges in London on June 30th was a world first.
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