Alltel has a 15 day return policy on phones activated on a new 2-year agreement. If you bought the phone at retail price, it's subject to the store's return policy instead. As for returning through the mail, I wouldn't advise it, since there are a lot of variables that could leave both you and the store out of a phone. Call the store first to see if they'll do it.

If you got it at a retail store, that's where you should return it for best results. If there's less than 10 minutes use, or 1MB of data used on it - the restocking fee should be waived.