Sometimes in a foreign country, your iPhone or iTunes will ask you if you want to update your carrier settings. You should choose ‘Yes’. The best explanation I’ve seen for this comes from Gary Rosenzweig of macmost.com. He explains carrier settings as being like printer drivers. If you connected your computer to another printer (printer B), you’d add new drivers to communicate with that printer. However, you wouldn’t think it would affect your ability to communicate with old printer (printer A) when you connect to it again, since it has its own drivers. It’s the same situation with phone carrier settings. When you update the settings on your mobile device, it enables your phone to communicate more effectively with the local phone network. It shouldn’t affect your phone when you return home to your normal network, though.