Unlock AT&T SIM card

Apple iPhone - Unlock AT&T SIM card

I have an iPhone 4 and will be traveling to New Zealand with it. The is a company there called 2 Degrees that offers a prepaid SIM card that will allow me to use their 3G network to make calls and get email, etc. Can I just put it into my iPhone replacing the AT&T SIM card or do I need to "unlock" it which I don't want to do!

iPhone ATT
Phone will probably need to have a carrier unlock.

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Also, the iPhone4 uses a micro-sim card, not the same one the rest of the world uses. Make sure they offer this smaller format. Regular SIMs can be trimmed AND I suspect 2 Degrees might not want you to do that.

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1. It will need a micro-SIM, not a standard one.

2. Why do you not want to unlock your phone?

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I called AT&T and they will not help me to unlock my IPhone! Big surprise! How do I unlock my phone and will it effect my apple warranty?