Buy your iPhone from the Apple Online Store and choose your own GSM carrier. You can change carriers at any time.
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Its easy to set up.
Get a compatible SIM (iPhone 3GS) or micro-SIM card (iPhone 4) and select a plan from the carrier of your choice. Then insert the card and set up your iPhone using iTunes on your Mac or PC. With a few clicks, you can start making calls.
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It works internationally.
Because this iPhone is SIM-free, you can purchase a micro-SIM or SIM card and service from a local carrier at your destination. Or check with your home carrier regarding international roaming charges.