Tired of having to put 555-01XX numbers in fiction just to avoid having it collide with a real phone number? Does the 555 look too obviously fake? Turns out the most invalid NANP phone numbers follow the format 19X-N11-XXXX, where X is any digit and N is either 0 or 1. This format violates all four rules NANP sets for valid phone number formatting, so it's a pretty good bet it will never become a valid phone number in the future either.
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Rule 1: the area code cannot start with 0 or 1. This is to prevent it from being interpreted as an international or long distance call prefix.
Rule 2: the second digit of the area code cannot be 9. I don't know why this rule exists, but it does!
Rule 3: the exchange code cannot start with 0 or 1 for the same reason.
Rule 4: the exchange code cannot end with 11 to avoid being interpreted as a well-known number, e.g. 911.
I would not be surprised if rules 2 and 3 disappear at some point, and possibly rule 4, but rule 1 will likely never disappear.
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Also 555-5555 is a valid phone number for a given area code, so uh, don't use that. Only 555-01XX is reserved for a given area code.
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Looks like the reason for rule 2, and to some extent rule 1, is so that they can extend the length of phone numbers in the future if needed. Interesting.
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