555/Headscratchers

Why didn't they use something less glaringly obvious, like, say, 729?

This is probably where Literary Agent Hypothesis comes in. You're supposed to imagine that the phones numbers in the universe of said work of fiction don't really start with 555. Even biographical works are known to use the 555.
- Consider it an in-joke turned mild Lampshade Hanging. People are just used to it.
555 numbers reach directory assistance or information, if they work at all, so there's little or no harm done if people decide to dial the number (as opposed to, say, 867-5309).
- It's a legal thing. Hollywood long ago reserved all the phone numbers ranging along 555-0something or other to 555-0something else or other so that they wouldn't run into any problems with idiots dialing up fictional phone numbers for fun. That kind of thing does happen. Any number of people in real life with the phone number 867-5309 have had to change their numbers because of nonstop barrages of crank callers asking for "Jenny".
- Yeah, but phone numbers are inexpensive. How much would it cost to reserve a few phone numbers and use them on a show/movie? They could even use pre-recorded messages or have some intern answer the phone in character.
- The number might not continue to be answered by some intern forever. Not to mention different area codes. 867-5309 was released in 1982, I don't think they'd still have an intern around to answer the phone, and people still get prank calls. The makers of Grand Theft Auto have some numbers they own and have set to a prerecorded message, but it's not always possible.
- Any number displayed with no country and area code will be tried everywhere - or at least everywhere local numbers happen to be that length. God's number in Bruce Almighty (page, 776-2323) was not in use in Buffalo (+1 716 776-2323) but it might be a radio station in Denver (+1 303), a news stand in Hull (+1 819) or a local church in yet some other area code. The DVD release changed the number to 555.
- This is why this trope is used. Note that Scrubs did do what was suggested with a real phone number to someone on set, and when the show ended, the number had to be taken out of service completely.
- And no, even 555 is not guaranteed to be free of individual subscriber's numbers... if dialled overseas. It's also (mostly) not reserved in area code +1-800. The reservations on 555 (information numbers), 950 (alternate long distance carriers), 959 and usually 958 (test numbers) are specific to the North American numbering plan. 123-4567 isn't valid in North America, but might be possible in other countries. Even within country code +1, codes which used to be safe (such as "area code 311") yesterday might dial City Hall today. The reservation on 0 or 1 in the second digit of North American numbers to indicate an area code was abolished in 1995 (so the "6060-842" which was a safe fake number to put on the B-side of the B52's "Rock Lobster" single at the end of the 1970's could well be somebody's working number today).

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