How to factory unlock iphone 4 for free?

I've got a locked iphone 4 from united kingdom and its carrier is T-mobile… Its updated to ios 7. Can anybody tell me how to unlock it for free so that i can use my local sim in it? And please no spams

Added (1). Or just simply unlock it for free…

You can't factory unlock any phone. Factory unlocked means believe it or not, it comes from the factory unlocked.

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