if i were to buy this phone: http://www.overstock.com/Electronics/RIM-BlackBerry-Storm2-GSM-Unlocked-Cell-Phone-Refurbished/6330382/product.html & i already have at&t would i be able to use it? i know it's already unlocked but would i have to do anything else once i get it to be able to use it , other than just putting my sims card in it ?