By Gary Holland
The 999 call originated in London in 1936 and was the world's first automatic telephone service to call the emergency services.
The Metropolitan Police Information Room during the 1930s.
A emergency number was suggested after a disaster in 1935 where five women died during a fire in Wimpole Street.
Neighbours dialling 0 for the switchboard found it jammed with calls and could not alert anyone of the fire.
Dialling 0 and asking the operator for police, fire or ambulance had been the recommended method since 1927.
Police stations during the 1920s and 30s were often receiving too many visitors alerting them to emergencies and having to field calls from the telephone - a new invention at the time.
Another method, for calling the police in an emergency, was to ask the operator for Whitehall 1212 - the Information Room set up at the Metropolitan Police's HQ at Victoria Embankment.
Information Room, circa 1960
The General Post Office, which ran the telephone network, proposed a three digit number that could trigger a special signal and flashing light at the exchange. The operators could then divert their attention to these priority calls.
In order to find the new emergency number in the dark or thick smoke it was suggested an end number was used so it could be found easily by touch.
111 was rejected because it could be triggered by faulty equipment or lines rubbing together. 222 would have connected to the Abbey local telephone exchange as numbers in the early telephone network represented the first three letters (ABBey = 222, 1 was not used due to the accidental triggering). 000 could not be used as the first 0 would have dialled the operator.
999 was deemed the sensible choice.
999 is world's oldest emergency call service
Launched in 1937 after five women died in London surgery fire
Initially a red lamp turned on and a klaxon siren sounded to alert operators when a call came in
First 999 mobile call made in 1986
Whitehall 1-2-1-2 was the first police emergency number
Europe's first telephone exchange opened in London 21 August 1879
The system came into place on 1 July 1937 covering a 12 mile radius from London's Oxford Circus. Several people have claimed to have made the first 999 call on the 2nd or 3rd July.
One early emergency call, reported in one newspaper to be the first, came on 8 July 1937 from Mrs Beard of Hampstead. She reported a burglar her husband was chasing and he was promptly caught.
During the first week there were 1,336 calls made to 999.
In November the same year the Information Room was able to take control of the calls. In 1938 the system was introduced to Glasgow.
Whitehall 1-2-1-2 was the original emergency number for the Metropolitan Police. The Information Room was set up at the Metropolitan Police's HQ at Victoria Embankment in 1934 to receive calls from the public and to transmit messages via radio waves to police vehicles.
A 1980s tape machine used to record 999 calls.
The police had been using wireless communications since 1922 and prior to that telegrams. In 1845 police arrested a criminal after using the telegram system and later in 1910 Dr Crippen was caught after telegrams between London, Canada and a ship in the Atlantic.
Morse code was another method employed by police to transmit and receive urgent calls.
In 1936 the Met Police were taking around 8,000 calls a month. Throughout that year 57,000 emergency calls were for the police, out of a total of 92,000.
By 1967 400,000 calls were made to the police in London with over a million for all services across the UK.
Today, emergency control centres are located around the country using the latest high tech equipment. There are three police control centres in London taking millions of calls each year from across the capital.
BT receives 30 million emergency calls a year - either to 999 or 112, the European emergency services number, which works in all European Union countries. There are strict procedures for handling such calls, set out in a code of practice between telecoms operators and the emergency services.