What do Ghostbusters, Back to the Future, and Good Will Hunting all have in common? A phone number.

Admit it: When you were a kid, and you saw someone on television write down their number as 555-XXXX, you tried calling it with your friends. Maybe you were up for a prank call, or maybe you really wanted to see who would pick up on the other line, but you’d only be met with disappointment as it turned out to be a fake number.

Why? That tricky 555 prefix.

The main reason movies and television shows used a 555-XXXX number is because, for decades, nobody was assigned a phone number beginning with 555—with the exception of an operator for directory assistance at 555-1212.

Other than that, all of the 555s were up for grabs in films.

Jamie Buchan, author of Easy as Pi: The Countless Ways We Use Numbers Every Day, guesses that the use of 555 started because, “The repeated digit may have made the combination memorable, which helped it gain traction. No major place names in the United States began with a combination of the letters J, K, and L (the letters assigned to the 5 key on a phone), the KLondike/KLamath prefix wasn’t exactly a coveted commodity.”

There is a movie called Last Action Hero in which the main character (a teenage boy) gets a magic movie ticket which transports him into his favorite action film. The movie he is transported into requires him to save the day with the help of Arnold Schwarzenegger, but first, he must convince Arnold that they’re in a movie and nothing is real. In order to do that, he asks a handful of passersby what their phone number is. Their answers all begin with 555. He then explains that the only people who have numbers with 555 are in the movies.

Last Action Hero isn’t the only movie with famous 555 numbers, though—there is a list of hundreds of entries. In fact, some famous films even use the same number as others on purpose.

So if you tried to get The Ghostbusters at their number (555-2368), you’d also be calling the hotel room from Memento, Jim Rockford of The Rockford Files, and Jaime Sommers from The Bionic Woman.

And how can you forget in Good Will Hunting, when Matt Damon holds up Skylar’s number against the glass as he says, “Well, I got her number. How you like ‘dem apples?” That number? 555-1294.

It was a good run for 555 numbers in film and television.

Numbers beginning with 555 were only used in Hollywood starting in 1973,  but that all changed in 1994.

Marcia Biederman, a writer for The New York Times explains, the run ended “when a contractor to the Federal Communications Commission, the North American Numbering Plan Administration, began accepting applications for these numbers from the public. They were meant to be dialed nationally or across broad regions of the continent, without regard to area code, on the model of 555-1212, the directory assistance number.”

Many of these 555 numbers were immediately snatched up by companies, all of which had hopes of using them as nationwide numbers; their plan was to have customers dial those number without an area code anywhere in the United States and get the same company. Unfortunately, it didn’t work out as intended.

The phone companies quickly realized that it would take away from the numbers available for 1-800 numbers and they began charging upwards of $2,500 per area code for the same 555 number. With over 250 area codes in the United States, that’s a lot of money, and these companies clamoring for 555 numbers weren’t happy.

The phone companies, however, insisted the 555 snafu wasn’t their fault and, instead, blamed it on the limits of technology.

Al Novell, a manager in Verizon’s federal regulatory group, explained, ”People have been coming to us over the past year or so and saying: ‘Do this for me. It’s easy. It’s inexpensive.’ But it isn’t.’ … A national 555 system would require broad application of Advanced Intelligent Network, which adds new services to existing switches, and would cost Verizon at least $108 million.”

So sure, members of the public (and companies) can request a 555 number, but that doesn’t mean much if you can’t use the number at all. As Biederman explains for the Times, “an assignment cost nothing, but it was left to the assignee to work out activation and payment arrangements with local phone companies.”

Mental Floss points out, “the 555 system still isn’t up and running in any meaningful way.”

As for numbers in film and television, a range of 555 numbers got specifically reserved for fiction. Shows and movies can use 555-0100 to 555-0199; beyond that, they can just use a real number to avoid seeming obviously fake.

In a fun turn of events, the movies and shows that do use real numbers often try to have some fun with it.

On Scrubs, Turk got a pager and told J.D to page him at 916-CALL-TURK.

When fans of the show dialed the number, they got a recorded message from Donald Faison (who played Turk) telling them the show has moved to a new time slot and to vote for them for the People’s Choice Awards.

Less fun though is when a movie doesn’t specify an area code. Woe to the person who has the same phone number in this situation. In Bruce Almighty, God’s phone number showed up on Bruce’s pager as a non-555 number. The number wasn’t in use in Buffalo, New York, where the set of the film was, but 30 people in different area codes around the country that did have that number got calls asking if they were God.

One woman was so incensed that she threatened to sue Universal Pictures because she was receiving at least 20 calls an hour; another man in Manchester, England, received upwards of 70 calls a day.

In subsequent releases, the film uses—you guessed it—a 555 number.

Another famous phone number continues to aggravate its owners: 867-5309.

In 1981, Tommy Tutone released their most famous hit, “877-5309/Jenny.” The casualties quickly began rolling in.

One junior high school received several hundred phone calls daily asking for “Jenny.” A 24-year-old administrative assistant received “hundreds of obscene phone calls.” A Chicago woman gave her phone number to a local radio station, WLS, which logged 22,000 calls in a single four-day period.

“When I heard we nearly melted a phone wire in Chicago, it made my day,” said Tommy Heath of Tommy Tutone.

As for the real Jenny—and yes, there was a real Jenny—she disconnected her phone (ignoring the lyrics of the song, which specifically ask her not to change her number).

“Friends of mine wrote her name and number on a men’s room wall at a bar,” said Jim Keller, the band’s guitarist and the song’s writer. “I called her on a dare, and we dated for a while. I haven’t talked with her since the song became a hit, but I hear she thinks I’m a real jerk for writing it.”

Granted, Keller didn’t give out Jenny’s area code, but he could have at least used a fake set of digits. Then again, “555-5555” doesn’t have quite the same ring to it.