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 Five Five Five. Even biographical works are known to use the Five Five Five.
 * Consider it an in-joke turned mild Lampshade Hanging. People are just used to it.
 * All 555 numbers reach directory assistance, 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.
 * 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.