The emergency telephone number is a special case in the country's telephone number plan. In the past, calls to the emergency telephone number were often routed over special dedicated circuits. Though with the advent of electronic exchanges these calls are now often mixed with ordinary telephone traffic, they still may be able to access circuits that other traffic cannot. Often the system is set up so that once a call is made to an emergency telephone number, it must be answered. Should the caller abandon the call, the line may still be held until the emergency service answers and releases the call.

An emergency telephone number call may be answered by either a telephone operator or an emergency service dispatcher. The nature of the emergency (police, fire, medical) is then determined. If the call has been answered by a telephone operator, they then connect the call to the appropriate emergency service, who then dispatches the appropriate help. In the case of multiple services being needed on a call, the most urgent need must be determined, with other services being called in as needed.

Emergency dispatchers are trained to control the call in order to provide help in an appropriate manner. The emergency dispatcher may find it necessary to give urgent advice in life-threatening situations. Some dispatchers have special training in telling people how to perform first aid or CPR.

In many parts of the world, an emergency service can identify the telephone number that a call has been placed from. This is normally done using the system that the telephone company uses to bill calls, making the number visible even for users who have unlisted numbers or who block caller ID. For an individual fixed landline telephone, the caller's number can often be associated with the caller's address and therefore their location. However, with mobile phones and business telephones, the address may be a mailing address rather than the caller's location. The latest "enhanced" systems, such as Enhanced 911, are able to provide the physical location of mobile telephones. This is often specifically mandated in a country's legislation.

History of emergency services numbers

When an emergency happened in the pre-dial (or "manual") telephone era, the user simply picked up the telephone receiver and waited for the operator to answer "number, please?" The user responded with "get me the police," "get me the Fire Department," or "I need an ambulance/doctor." Even in a large city it was seldom necessary to ask for these services by number.

In small towns, operators frequently went the extra mile by knowing where to reach doctors, vets, and law enforcement personnel at all times. Frequently, the operator was also responsible for activating the town's fire alarm.

When manual switching systems began to be replaced by automatic, or "dial" systems, there was frequently concern among users that the very personalized emergency service provided by manual operators would be lost.

This problem was at least partially solved in the USA, Canada, and the UK by dialing "0" for the local assistance operator in case of emergency, although faster service could be obtained if the user dialed the full number for the Police or Fire Department. This system remained essentially unchanged throughout most of North America until the 1970's.

The first emergency number system to be deployed was in London on June 30, 1937. When 999 was dialed, a buzzer sounded and a red light flashed in the exchange to attract an operator's attention. It was gradually extended to cover the entire country, but it was not until the late 1970s that the facility was available from every telephone.

In the days of loop disconnect dialing, attention was devoted to making the numbers difficult to dial accidentally by making them involve long sequences of pulses, such as with the UK 999 emergency number. This contrasts to modern times, where repeated sequences of numbers are easily dialed on mobile phones, particularly as mobile phones will dial an emergency number while the keypad is locked or even without a SIM card. Some people in the UK have reported accidentally dialing 112 by loop-disconnect while working on extension telephone wiring, and point to this as a disadvantage of that number. [citation needed]

The first North American emergency number was the 999 system deployed in Winnipeg, Manitoba, Canada in 1959 at the urging of Stephen Juba, mayor of Winnipeg at the time. The first US 911 emergency phone system was set up in Alabama in 1968, but it was not in use everywhere until the 1980's to standardize the number across most of the NANP, Canada switched to using 911 as its emergency number in 1972. (Some Caribbean islands use 999.)

The implementation of 911 service in the USA was a gradual and haphazard process. The chief obstacle was the fact that telephone service boundaries seldom, if ever coincide exactly with governmental and other jurisdictional boundaries. In other words, a user might dial 911, only to discover that he had been connected to the wrong dispatch center because he had telephone service from one location, but lived within the boundaries of another jurisdiction.

Furthermore, a great deal of electromechanical switching equipment was still in use, and much of it was difficult if not impossible to adapt to recognize 911, especially in small towns and rural areas where the call might have to be switched over a considerable distance. For this reason, it is still not unusual for a County Sheriff's Department to have an "800" (long distance, toll-free) number.

Gradually, various problems were overcome; "smart" or "enhanced" 911 systems were developed that not only displayed the caller's number and address at the dispatch center, but also could be configured so that 911 calls were automatically routed to the correct dispatch center, regardless of which central office the caller was served from.

The rapid replacement of electromechanical switching systems in the 1980's with electronic or digital systems eliminated the problem of older switches that would not recognize 911. At this point, 911 service is available in most of North America, but there is still the occasional small, remote town that does not have it.

In France, in 1928, telephone operators had to connect the calls for emergency reasons even when the phone service was closed. In 1929, an automatic connection system was set up, initially for fewer than 10,000 people in Paris, allowing them to dial 18 to reach the fire brigade. The service was not widespread until the 1970s.

The CEPT recommended the use of 112 in 1972. The European Union subsequently adopted the 112 number as a standard on 29 July1991. It is now a valid emergency number throughout EU countries and in many other CEPT countries. It sometimes works in parallel with other emergency numbers in countries such as Britain and Ireland.