A telephone number or phone number is a sequence of numbers use 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.