Universal emergency telephone number system
A means by which a user of a telephone and request assistance from a variety of emergency services, including police, firefighting, emergency medical service, and perhaps others appropriate to the location and user, such as search and rescue. While there has not yet been worldwide standardization on the number to reach the dispatcher at the Public Safety Answering Point (PSAP), the most common are 911 and 112.
Caller location
Enhancements of the service give additional information to the dispatcher, such as the location of the calling telephone. Even for a plain old telephone service (POTS) user, location reporting is not a trivial problem, at least for anything more complex than a single-family house. If the caller is in a business with many extensions, or an apartment building, the location provided may be no more precise than the street address. One enhancement gives a more specific location in a multiple-unit building.
It is required that most new cellular telephones be manufactured with a GPS receiver, which can send the actual location of the caller. In many telephones, however, the user can completely disable GPS, or limit position reporting only to emergency calls.
With voice over Internet Protocol, the user could be anywhere that has Internet connectivity. Most VoIP services administratively assign a physical address to be reported with emergency calls.
PSAP location
911 or 112 is not the physical location of a telephone, but a code that tells the first telephone end office to switch the call to an appropriate PSAP. Unfortunately, many such systems are not sufficiently fine-grained. For example, many systems in the United States have a three-digit area code, a three digit local exchange, and a four digit local line number. The local exchange usually is associated with a particular physical area, but that area may not correspond to the jurisdiction of individual emergency services. For example, (703)998-XXXX in the state of Virginia, in the United States, contains telephones in Alexandria and Arlington. All 911 calls, at least within recent times, would go to the Alexandria PSAP, and, if the emergency was in Arlington, the Alexandria dispatcher had to transfer the call to Arlington.