I moved to the US from Europe a few months ago. I have an unlocked Phone 6s Plus that brought with me that would only work with AT&T and not Verizon. Now I'm going to buy a 7+ and I'm considering switching to Verizon (the LTE speed in New York is just awful), but I don't understand the differences of the models Apple sells: if I just go to an Apple Store and get a SIM free iPhone, does it mean that I can use it with AT&T, my SIM from my european carrier (Vodafone) and Verizon? If I get the Verizon model at the Apple Store, is it locked to that carrier, or is it compatible with AT&T? Thank you in advance.