Can I use an iPhone 4 with straight talk?

I want to use my gamestop credit to buy an iphone from them, but its an at&t iphone. Would I be able to switch to straight talk somehow? I've heard of people using straight talk sim cards, but don't know if its true. The only iphone for straight talk I know of is the one that they offer. And what is unlocking and how do you unlock a phone?

The AT&T iPhone 4 is compatible with Straight Talk's BYOD SIM card plan. Unlocking is what is done to allow a phone to work on a different carrier. If the phone has a clean ESN/IMEI, AT&T should unlock it free, although there are paid unlocking services available nationwide.

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