I've heard one can get a bit more if an iPhone is unlocked before selling it back, say, to Amazon (etc.) My understanding is that one can contact AT&T and they will unlock the iPhone. (I've since moved on with a new iPhone 6s with a two year contract. My old iPhone, a 5s, is no longer under contract nor has cellular service.) What does "unlocking" do if one is supposed to pull out and keep the AT&T SIM card before selling an iPhone? Is this what you'd recommend doing, or is it not worth it and should I just sell it back to Amazon as a locked AT&T phone?