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 become longer. In addition to telephones, they now access 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, 0-0-0, 4-1-1, 9-9-9 and 1-1-2, 111 being the New Zealand Emergency Services number and 000 for Emergency services in Australia. 911 is the Emergency Services number used in Canada and the United States).

Many systems also allow calls within a local area to be made without dialing the local area code. For example, a phone number in North America will start with three numbers (such as 918), which is the area code, followed by seven digits split into sections of three and four (such as 555-1212), which is the local number.

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. Some mobile phones 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.