Hi, I purchased two iPhone 6+ with 2 years ATT contract and two Tmobile iPhone 6+. My plan is sell my ATT contracted iPhone and use the Tmobile phone. I would like to sell the ATT phones new seal in box either craiglist or ebay. Hence, the new sim card will be inside. Will that create a problem when the buyer activate the att phone? I certainly do not want some stranger start taking over my phone line.