Travel expert Rick Steves offers a lucid explanation of the complicated business of using a cell phone in Europe. Europe uses a transmission standard called GSM (Global System for Mobile Communications). In the U.S., most companies operate on a network called CDMA (Code Division Multiple Access), which doesn’t work abroad. The exceptions are T-Mobile and AT&T, which do use GSM.
GSM phones hold a little chip behind the battery. Called a SIM (Subscriber Identity Module) card, it contains the phone’s memory, including its number and location. An American-based GSM phone, for example, on the AT&T network, could work in Europe. But because the SIM card is U.S.-based, it would rack up international calling rates anywhere but home. Not only that, every time you cross a border in Europe, you get smacked with “roaming” charges. To avoid these, you must literally buy a new SIM card for every country that you visit.