A telephone number is a sequence of decimal digits (0-9) that is used for identifying a destination telephone or in a telephone network. In fact, not only telephones but also faxes, modems, subscribers and network services can have telephone numbers. Each such endpoint must have a unique number within the network, and the number of endpoints determine the necessary length of the telephone number. It is also possible for each subscriber to have a set of shorter numbers for the endpoints most often used. These "shorthand" numbers are automatically translated to unique telephone numbers before the call can be connected.
Most telephone networks nowadays are interconnected in the international telephone network, where the format of telephone numbers is standardized by ITU-T in the recommendation E.164, which specifies that the entire number should be 20 digits or shorter, and begin with a country prefix. In most countries, this is followed by an area code and the subscriber number, which might consist of the code for a particular telephone switch. ITU-T recommendation E.123 describes how to represent an international telephone number in writing or print, starting with a plus sign ("+") and the country code.
Before a telephone call is connected, the telephone number must be dialed by the calling party or caller. The receiving party or callee might have equipment that presents caller ID before the call is answered.
History: After Alexander Graham Bell invented the telephone in 1876, it was soon used as a subscription service with the invention of the telephone switch or central office. Such an office was manned by an operator that connected the calls by personal names.
The latter part of 1879 and the early part of 1880 saw the first use of telephone numbers at Lowell, Massachusetts. During an epidemic of measles, Dr. Moses Greeley Parker feared that Lowell's four telephone operators might succumb and bring about a paralysis of telephone service. He recommended the use of numbers for calling Lowell's more than 200 subscribers so that substitute operators might be more easily trained in the event of such an emergency. Parker was convinced of the telephone's potential, began buying stock, and by 1883 he was one of the largest individual stockholders in both the American Telephone Company and the New England Telephone and Telegraph Company.
(note: a lot of the below probably applies to the entire North American Numbering Plan, i.e. US, Canada, several countries in the Carribbean, not just the United States)
Aside from access codes, such as outside line, long distance, or international calling, a telephone number in the United States consists of three parts. An example might be 303-555-1212.
The area code or NPA (Numbering Plan Area) is a three digit number (the 303 above) which allows the system to route calls into a small geographic area. NPAs are defined by the North American Numbering Plan, and administered by Bellcore (do they still do this??).
Within each area code are individual exchange numbers, represented by the second three digits, 555 in the above example. This usually represents a specific central office switching machine (or telephone switch). This local machine may actually operate with several exchange numbers, such as 555, 554, 553 and 552. If it is large enough, it may also switch additional exchange numbers, such as 54x and 53x. Exchange numbers are also known as NXXs - not an abbreviation but a code where N=2..9 and X=0..9.
The final four digits are the subscriber number, 1212 in the example above. This number connects to a specific telephone circuit within the exchange.
It is traditional in American television and film to have all phone numbers begin with 555, as in 555-3245 (also known as KLondike-5 in older movies). This exchange number is typically unused by phone companies, avoiding innocent people being bombarded with unwanted phone calls. An exception is the number 555-1212 which is directory service in the US for each area code, it is basically the 411 service outside the local area code. Some regional services such as PacBell in California are now (as of 2001) converted to provide nationwide directory service via the 411 numbers which makes the (???) 555-1212 number unnecessary within California.
Some common numbers in the US system:
0 - operator assist 00 - Long Distance Operator Assist 311 - non-emergency police matters 411 - local directory service 611 - telephone line repair service 911 - emergency dispatcher for fire, ambulance, police etc. (area code) 555-1212 - non-local directory service. 767-2676 (or POP-CORN) or 767-8900 - local time reporting service (may not be available in all area code) 211-2244 - self identification, report phone number of the caller
Note that not all countries use the same numbering system. For example, the emergency number in Hong Kong and the United Kingdom is 999 instead of 911, and in much of Europe it is 112.
Presently, telephone numbers in Australia consist of a single digit area code and eight-digit local numbers, the first four of which generally specify the exchange, and the final four a line at that exchange. (Most exchanges though have several four-digit exchange codes.)
Australia is divided geographically into a few large area codes, frequently covering more than one state. The long distance prefix is '0', while the main international prefix is '0011' (there are others for special purposes, such as charging in half-hour blocks).
Prior to the introduction of eight-digit numbers in the early to mid-1990s, telephone numbers were seven digits in the major capital cities, with a single digit area code, and five digits in other areas with a two digit area code.
Australia also the free call area code 1800. This is copied from the U.S. prefix 1-800, but while in the U.S. the '1' is the long-distance prefix and '800' is the area code, '1800' in Australia is itself an area code (prior to the introduction of 8-digit numbers, the area code was '008'). Similarly, '190x' is the area code for charging services (i.e. recorded information, competition lines, psychics, phone sex, etc.). There are also '13' numbers, which work across large areas (up to across Australia) and only charge a local call, routing the call to the appropriate place in a given area. (For example, a company could have the number 139999 and have the telephone company set it up so that calls made in Melbourne would route to their Melbourne number, calls made in Brisbane to their Brisbane number, and calls made anywhere else in Australia route to their Sydney number, all at a local charge cost to the caller.)
Mobile phone numbers have three digit area codes, e.g. 412. The area codes are allocated per network, although with the introduction of number portability there is no longer a fixed relationship between the area code a mobile is in and the network it is attached to.
'000' is the emergency number, but the international emergency number '112' works as well (at least on mobile phones.)
Phone numbers within Australia are allocated by the Australian Communications Authority.