I want to try and take advantage of the M$ Live 25% off via online purchase. I was planning on going into an AT&T store in person and buying it there. I want to put my phone on my sisters family plan because she gets 24% off from her work. If I buy the phone online will I get hit a full months phone and data plan? Lets say I put it on my sisters plan the day I receive and activate the phone. Anyone have any insight on how this would play out?