Avoiding Roaming on the iPhone
iPhone users usually pay a set monthly price for mobile data services, but this changes as soon as you take your iPhone roaming overseas. Whenever you travel to a different country, it is possible to run up a huge bill if you aren't careful. The reason that the overseas charges are so much higher is that you are no longer using the data network that belongs to your wireless carrier.
Your phone will automatically go into data roaming mode, which means you will no longer be using your data plan and will instead pay per megabyte of data sent or received by your phone. This can get expensive very quickly, and it's possible to rack up bills of hundreds or even thousands of dollars on a short vacation. The easiest way to avoid this fate is to simply turn off your data roaming.