To make a long story short, I have two iPhone 4S. One is with Verizon on an unlimited data plan and the other is on AT&T with a 2 gb data plan. My Verizon iPhone is a 64GB model. I tried putting my AT&T sim card into my Verizon phone and it worked, but I got an error from iOS that said my sim card was not valid, but I was receiving full bars from AT&T instead of Verizon. AT&T has much faster data speeds, and instead of having two phones, I'd like to switch out sims when needed and return my AT&T phone. Is this possible?