Can AT&T stores give me a replacement???

JCMA90,

All warranty service is handled by Apple. You can call AppleCare and have a box overnighted to send your iPhone in for service.

You will have the option of essentially renting a loaner phone during the repair.

There is more information in the iPhone Service FAQ:
http://www.apple.com/support/iphone/service/faq/

Hope this helps,

Nathan C.

Now I would love to stress that being it says you are from Mexico City, Since Apple does not allow unlocking and voids warranty.
If this is the case:

1) Apple will not send a loaner to a non US or Apple apprived country most likely
2) Once Apple notices that your phone is unlocked, you will be SOL for a free warranty repair
3) You can take it to a local Apple Store in the US (Mission Tx), but this will call your warranty into play yet again if it is not attached to a valid account.

This issue sounds most likely like unlocking software, messing with your iPhone

If the above is true, I am sorry to say but your phone is most likely a brick and you may be SOL

Best regards

no sorry AT&T can not

Either call

1-800-myiphone (they will overnight you a free loaner)
or
visit a Apple store

Where did you order it from?

Yes. That person will be FedEx. They will send you a loaner/replacement and you will ship them back your phone.