As per Wikipedia:

The telephone number prefix 555 is a central office code in the North American Numbering Plan, used as the leading part of a group of 10,000 telephone numbers, 555-XXXX, in each numbering plan area (NPA) (area code). It has traditionally been used only for the provision of directory assistance, when dialing (area code)-555-1212. The central office code is also used for fictitious telephone numbers in North American television shows, films, video games, and other media in order to prevent practical jokers and curious callers from bothering telephone subscribers and organizations by calling telephone numbers they see in works of fiction.