If you are only coming for a visit it's going to cost minimum $50USD for one of the phone stores to unlock your phone. At least that is what I charge the store and then they will upcharge you on their end to make a few bucks. Chances on that unless you have a factory unlocked iPhone4 then your phone is probably not unlockable. Only the first batch of iPhone4's are unlockable. On the phone go to Settings, then General, then About. Scroll down and if it says anything other than 01.59.00 then you can't be unlocked. Only 1.59.00 is unlockable with a software unlock and if you have ever updated the iPhone4 it's not on that software.

I'd follow the advice of the others and just get a second cheap phone and leave the iPhone4 at home or keep it off while you're here. If the phone is even just powered on and you receive a voicemail you will incur extra charges on your bill.

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