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If you’ve been eagerly waiting for iPhone Dev Team to release a software solution to unlock your iPhone then may have also heard about a SIM card hack called Gevey SIM.
Gevey SIM Interproser doesn't need users to jailbreak their iPhone but works by forcing the activation of the baseband using the emergency dialer. It can unlock iPhone 4 basebands 04.10.01 (bundled with iOS 4.3, iOS 4.3.1), 03.10.01 (bundled with iOS 4.2.1), 02.10.04 (bundled with iOS 4.1).
This is how it works: