When traveling overseas with your iPhone beware of huge surcharges for using international calling and data roaming if you don't have a plan that covers you while your abroad. There are several examples of people coming back from a trip and find bills of hundreds or thousands of dollars in data roaming (from checking their email, browsing, etc.) charges from their carrier. They assumed that their data plan covered them.
For more information, check out the following article from AT&T
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Notes/warnings: If you do have an international data plan that covers you while traveling, make sure that you understand its limitations and restrictions. Use this tip at your own risk. If you don't have international data plan I would not recommend turning off this feature.To turn off your iPhone's data roaming feature, that prevents the phone from using foreign service providers for sending/receiving data. From Settings select General > Network > Data Roaming, and turn the feature off.
For more information, check out the following article from AT&T