Hello! @David018 thanks for posting your phone number change questions.
You are not required to change your number if you move to another area code, but should you choose to do so, the fee is 40.00 and will be added to your bill in 1-2 bill cycles. You can change the number by logging into your online account, then hover over My AT&T at the top of the page and click on Account Overview. From the I want to… drop-down list, select Manage my plan & services, and then select Change a wireless number. Select the How would you like to search for a new mobile number? Then select next. Select the desired number then next to change it.
Hope this helps!
Charise