The most telephone numbers we see in the US television shows, movies, and fiction literary start with 555. These are fictitious numbers used to prevent prank calls to real people as there are many cases of people calling the numbers seen in the movies.

A demonstrative example of this situation is with the film Bruce Almighty. The makers of the movie chose not to use the prefix, and people whose phone numbers matched one mentioned in the film received many calls from callers asking for "God".

There are real numbers with prefix 555, but numbers 555-0100 through 555-0199 were specifically reserved for fictional use some years ago. Recent films, however, do not always heed the limits, and some producers even acquire actual telephone numbers solely for the purpose of using them in the movie or on TV.

For instance, on show Scrubs the surgeon Chris Turk gave out his phone number: 916-CALL-TURK. If you call the number, you'll hear a message from one of the characters.

In older times the prefix 555 went largely unused, since in the old exchange-name telephone number, no English place names contained the combination of the letters J, K, and L – all assigned to the digit "5" on the phone. So, the phone companies started encouraging the TV and movie producers to use the 555 prefix for fake telephone numbers.

There is also another explanation, which may well be an urban myth. Allegedly, decades ago there was a branding deal between MGM and the cigarette company 555 over the use of the digits in movie telephone numbers with the purpose of creating mass market brand recognition for the cigarette brand.

See the 555-LIST, an ongoing project to compile all the 555 numbers used in TV and movies.