How to buy an unlocked iPhone 6s?

My old iPhone just broke and I need a new unlocked phone As soon as possible. I'm looking at the apple website and it does not show any unlocked option for the 6s. The only options that it shows are Verizon, AT&T, Sprint, and T - Mobile. I have AT&T as my carrier, so I clicked on the AT&T option, but it says pay in full. Does that mean that it is an unlocked phone?

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Yes. An iPhone paid in full comes unlocked.

If you buy the phone without contract paying full retail from Apple Store, they'll unlock the phone. Alternatively, Verizon iPhones come unlocked by default so you can just go ahead and buy Verizon iPhone 6S and it will be readily available for using on AT&T. If you need unlocked iPhone 6S at reasonable price with bundled offer, mail me for more info.