Is an iPhone Dual-Voltage?
Apple's iPhone is a world phone capable of being used all around the globe, both because its cellular chip supports all networks and also because of other features, such as its compatibility with different power voltages used around the world. Use the iPhone with plug adapters or, if you can, with different plugs entirely.
Purpose of Adapters
Irrespective of differences in voltage -- the United States uses 110-volt outlets, while Europe and Asia use 220-volt ones -- power adapters allow you to plug devices that use differently sized or shaped prongs into outlets that don't otherwise fit them. Power plugs for American adapters, for example, use flat, long prongs, so to plug such a device in a European wall outlet you need to use an adapter with two round prongs.
Obtaining New Accessories
If you plan to visit or live abroad for a long period of time, one way to avoid having to use an adapter each time you plug the device in is to purchase new accessories from an Apple store or a third-party retailer. Since the iPhone is capable of charging with both 110 volts and 220 volts, new accessories solve the problem at hand, which is plug shape compatibility.
Roaming and Service
After you've resolved power issues, it's important to take service issues into account. Namely, you must pay an increased roaming rate when you use the iPhone in other countries with your American service plan. Although some U.S. carriers offer affordable international rates, you may do best visiting a local provider who can unlock your device for use with one of its subscriber identification module cards.
The iPhone's warranty applies internationally, meaning that any damage you incur while traveling abroad is covered, so long as your use of the device falls within Apple's intended uses. Electrical shocks incurred during charging aren't covered, as they're considered accidental, so always use a surge protector with the iPhone when you can to avoid accidental power surges.