A telephone number is a sequence of decimal digits (0-9) that is used for identifying a destination telephone line in a telephone network. Telephone numbers are often assigned to lines that have other devices hooked to them such as faxes, modems, subscribers and network services. Each such endpoint must have a unique number within the public switched telephone 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. Some special services have their own short numbers (e.g. 9-1-1 and 4-1-1).
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 or city 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. Telephone numbering plan
Before a telephone call is connected, the telephone number must be dialed by the calling party or caller. The called party might have equipment that presents caller ID before the call is answered.
Businesses used to have a single telephone number for the main switchboard of that business and a switchboard operator would connect the call within the business. If the called party didn't answer, the caller was typically transferred back to the switchboard. With voicemail and more technology, businesses now use Direct Inbound Dialing (DID) lines so that outside callers can call to a specific person in a business. Often times, the DID number uses a pattern from the called party's telephone internal extension. For example, within Smith Company (fictitious), a caller may dial 225 to reach Mr. Smith, but an outside caller may dial 448-9225 to reach Mr. Smith (with the last three digits symbolizing Mr. Smith's extension).
After Philipp Reis invented the telephone in 1863,  (http://news.bbc.co.uk/1/hi/sci/tech/3253174.stm) Alexander Graham Bell commercialized it 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.
Even after the assignment of numbers, operators still connected most calls into the early 20th century; "Hello, central, get me Underwood-342"
Connecting through operators or "central" was usual until mechanical dialing of numbers became more common in the 1920s.
In North America, the digits 2-9 of phone numbers were alloted 3 letters of the alphabet apiece.
Phone numbers were not usually strictly numeric until the 1950s. From the 1920s until then, most urban areas had "exchanges" of two letters, followed by numbers. This was considered easier to remember, although in the later part of this period it required the memorization of 7 characters (the same number of characters as is usual for local calling in 2003). A word would represent the first two digits to be dialed, for example "TWinbrook" for "89" ; "BYwater" for "29"; 736-5000 was "PEnnsylvania - 6- 5 thousand".
Phone numbers were traditionally tied down to a single location, but the introduction of mobile telephones has changed this. In many countries, the practice of number portability allows customers to transfer a phone number from one local exchange carrier to another, or even from a fixed-line phone to a mobile phone.
- ITU-T Recommendation E.123: Notation for national and international telephone numbers, e-mail addresses and Web addresses (http://www.itu.int/rec/recommendation.asp?type=items&lang=e&parent=T-REC-E.123-200102-I)
- RFC 2806: URLs for Telephone Calls (http://www.ietf.org/rfc/rfc2806.txt)
- International Telephone Directories (http://numberway.com/)
- History of UK dialling codes, with lists of codes and more links (http://www.rhaworth.myby.co.uk/phreak/tenp_uk.htm)
- The Telephone Exchange Name Project (http://ourwebhome.com/TENP/TENproject.html)