Telephone numbering plan

A telephone numbering plan is a type of numbering scheme used in telecommunication to assign telephone numbers to subscriber telephones or other telephony endpoints. Telephone numbers are the addresses of participants in a telephone network, reachable by a system of destination code routing. Telephone numbering plans are defined in each of administrative regions of the public switched telephone network (PSTN) and they are also present in private telephone networks.

Numbering plans may follow a variety of design strategies which have often arisen from the historical evolution of individual telephone networks and local requirements. A broad division is commonly recognized, distinguishing open numbering plans and closed numbering plans. A closed numbering plan imposes a fixed number of digits to every telephone number, while an open numbering plan allows variance in the numbers of digits. Many numbering plans subdivide their territory of service into geographic regions designated by an area code, which is a fixed-length or variable-length set of digits forming the most-significant part of the dialing sequence to reach a telephone subscriber.

555 (telephone number)

Telephone numbers with the prefix 555 are widely used for fictitious telephone numbers in North American television shows, films, video games, and other media.

Not all numbers that begin with 555 are fictional—for example, 555-1212 is one of the standard numbers for directory assistance throughout the United States and Canada. In fact, only 555-0100 through 555-0199 are now specifically reserved for fictional use; the other numbers have been reserved for actual assignment, with the notable exception of the 800 area code where only 800-555-0199 is reserved. The entire 555 exchange is reserved in all overlay North American toll-free area codes (844, 855, 866, 877, 888) and in Canada's rarely used non-geographic area code 600.

Area code 555 in the North American Numbering Plan is reserved for Directory Assistance applications.

Fictional usage

Telephone companies began encouraging the producers of television shows and movies to use the 555 prefix for fictional telephone numbers by the 1960s. Two early examples include The Second Time Around (1961), which used 555-3485, and Panic in Year Zero! (1962), which used 555-2106. "Rossmore 555", mentioned in Eyes in the Night (1942), may be considered a precursor. In television shows made or set in the mid-1960s or earlier, "KLondike 5" or "KLamath 5" reflects the old convention for telephone exchange names.

Telephone number

A telephone number is a sequence of digits assigned to a fixed-line telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices for data transmission via the public switched telephone network (PSTN) or other private networks. Most telephone numbers are assigned to one telephone line or one mobile telephone, and most lines or mobiles have one number.

A telephone number serves as an address for switching telephone calls using a system of destination routing. Telephone numbers are entered or dialed by a calling party on the originating telephone set, which transmits the sequence of digits in the process of signaling to a telephone exchange. The exchange completes the call either to another locally connected subscriber or via the PSTN to the called party.

The use of telephone numbers instead of subscriber names to indicate to the telephone operator what destination line a caller wished to be connected to was developed and first used in the autumn of 1879 in Lowell, Massachusetts during a measles epidemic. Moses Greeley Parker, a local doctor, realized that if all four of the city's operators were incapacitated by the epidemic, their replacements would have great trouble quickly learning which of the switchboard's 200 jacks were assigned to which subscribers. He recommended the use of numbers instead. "The local Bell company management at first protested that its customers would consider their designation by numbers to be beneath their dignity; nevertheless, it saw the logic of the doctor's suggestion and followed it. The subscribers were not outraged; the epidemic quickly passed, but telephone numbers did not."