A lot of people are running into a problem when porting their old cell number to AT&T so that they can continue to use the same number with their iPhone. The iTunes registration software and customer service reps will say that the nummber cannot be transferred because it’s ineligable or that the other carrier wouldn’t release the number.
AppleInsider discovered that the problem stems from some obscure rule that doesn’t allow you to transfer a number if your zip code does not match the region of your old number’s area code. This is common for people that have moved and retained their old cell number — even if they moved within the same state.
In the case of the iPhone registration process, the solution is simple:
When the registration software asks for your zip code, just enter the zip for a region where your cell number’s area code will match. Then, make sure to enter the correct information for the billing address, but when it asks you for an address where the phone will primarily be used, choose other (instead of same as billing) and enter a bogus address with the zip code that matches your number’s area code. This should let your number be transfered, and you can correct the bogus address later.
How to port ‘ineligible’ mobile numbers to AT&T and iPhone – Link