I'm planning on going to the Apple Store tomorrow to start a T-Mobile plan and preorder the 6 Plus. I currently have AT&T on an iPhone 5, and another spare iPhone 5. I need to stay on AT&T until I can actually open and activate the 6 Plus. My plan is to trade in my current iPhone 5 to cover the cost of the $99 64 GB 6 Plus I want. If I were to do so and switch to T-Mobile, would the AT&T SIM card in my main phone still work with the same number in the spare phone on my other line? I guess another way of asking the question is this- Am I right in thinking that my number won't switch over to T-Mobile until the 6 Plus is opened and activated? I have never switched carriers before so this is all new to me- pardon me if I sound uninformed and stupid. Definitely guilty of the former, probably guilty of the latter.