What’s to stop some area code from containing the famous pseudo-prefix 555? Would all fives screw with the machinery used (unlikely), or is there a convention in place (more likely)? If there is a convention, why isn’t it 123 or 666? Why 555?
Cecil’s column in which he addressed this issue:
http://www.straightdope.com/classics/a1_374a.html
Unca Cecil says in the column that 555 is reserved for directory assistance; I have also heard that it is reserved for phone company offices and the like. (area code) 555-1212 is the universal directory assistance number, if you feel like trying it out.
Everything you could possibly want to know about telephone numbers:
http://www.atis.org/atis/clc/inc/incdocs.htm
555? WHy not call it in your area & see what happens. Chances are youll get a busy signal. That’s why they use it in films.
When I was a teenager in the 80s, my family operated a pub in a small country town. The number of the pub began with 555.
OK, I live in Australia, so it’s not really relevant, but I remember thinking it was way cool at the time.
There are, however, a few poor unfortunate souls with the surname “Batman” (quite a historic name in the Melbourne area, BTW - Redboss might be able to tell you about it). Those guys have been known to receive the odd crank call or two.
I’ll assume you are talking about North American phone numbers.
Two main reasons for no area code 555:
-
As MsWhatsit said, they* don’t want to assign area code 555 because it could be confused** with exchange cose 555 used for directory assistance, as in 555-1212.
-
All area codes with the second and third digits identical are called ‘easy-to-remember’ codes, and are reserved for special services (usually not assigned according to geographic location).
The oldest and most famous of these services is toll-free calling , whose numbers started with area code 800, and now encompass 888, 877, 866 and 855 as well.
There was an area code 500 assigned for ‘follow-me’ numbers, which could be programmed to ‘point to’ different numbers at fifferent times of the day, as the subscriber desired. (9AM to 5PM, the office. 5PM to 6PM, the cellphone. 6PM to 9PM, home. 9PM to 9Am, voicemail. Like that.) This was going to be expanded with area code 533, and presumably others in the 500 series, but for some reason the service was never promoted and/or never became popular. If it had taken off, possibly 555 would have been used for these numbers eventually.
- Hi Opal!
When Nevada’s area code 702 split, they*** wanted to get area code 777 (Lucky Sevens!) for Las Vegas. NANPA disallowed this, because it would have been a regular geographic area code. Vegas retained 702, and the rest of Nevada got 775.
At this time, Los Angeles was undergoing a lot of area code additions, and the joke was that “if Vegas got 777, there should be no problem assigning 666 to LA…” 
*‘They’ are the North American Numbering Plan Administration, http://www.nanpa.com/, and the various national numbering authorities that the NANPA coordinates.
**Numbering authorities dislike to assign numbers with area code and exchange code identical: they say it could lead to confusion.
As an example, suppose that you live in area dode 496. Area code 456 has an exchange 797, and has local dialing with area code 797. You start dialing a number with ‘797’. Are you making a local call to exchange 797 in your area code, or a local call to the neighbouring area code 797?)
However, increasingly-large areas of North America require that all ten digits of the phone number be dialed for every call, which removes this confusion, because all dialed numbers include the area code.
***This ‘they’: not NANPA; presumably the Vegas gambling-and-tourism promotion people.
I haven’t read the column and I’ll look it up, but my pager number begins with that prefix (303-555-xxxx).