A telephone number or phone number is a sequence of numbers used to call from one telephone line to another in a telephone network. When telephone numbers were invented, they were short - as few as two or three digits - and were used by people to call a few neighbors. As phone systems have grown and interconnected to encompass the world, telephone numbers have grown longer and more complex, and in addition to telephones they now designate many other devices, such as computers and fax machines.

The number contains the information necessary to identify uniquely the intended endpoint for the telephone call. Each such endpoint must have a unique number within the public switched telephone network. Most countries use fixed length numbers (for normal lines at least) and therefore the number of endpoints determines 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" or "speed calling" 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, 4-1-1, 9-9-9 and 1-1-2). Many systems also allow calls within a local area to be made without dialing the local area code. Example A phone number in North America will start with three numbers (918) which is the area code, followed by three more (555) and four more after that (1212)

Most telephone networks today 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 15 digits or shorter, and begin with a country prefix. For 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. When calling an international number from a fixed line phone, the + must be replaced with the international call prefix chosen by the country the call is being made from. Mobile phones generally allow the + to be entered directly.

The format and allocation of local phone numbers are controlled by each nation's respective government, either directly or by sponsored organizations (such as NANPA overseen by NeuStar Inc.) See 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.

Business phones

Businesses in the middle 20th century and earlier had a single telephone number for the main telephone switchboard of that business. 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 Inward Dialing (DID) (Direct Dialling Inwards (DDI) in the UK) lines so that outside callers can call to a specific person in a business. Often, the DID number uses a pattern from the called party's telephone internal extension. For example, within the Acme Corporation, 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 representing Mr. Smith's extension). Some companies arrange their dial plan to restrict the DDI capability to certain parts of the PABX or Centrex numbering range: Mr Smith on 225 might have it, but Mr Jones on 374 or Mr Brown on 427 would not. If a merger has happened in the past there may even be different exchange codes mapping to different parts of the internal numbering range. DID became an important feature of Centrex.