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Phone Numbers from WikiPedia:

A telephone number or phone number is a sequence of numbers used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, 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.

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