Only unlocked cell phones that are GSM-enabled have the ability to work in the U.S. GSM, which is short for "Global System for Mobile Communications," is the predominant cell-phone technology system in Europe. In the U.S., only two companies use GSM technology: Cingular and T-Mobile. In Europe, the majority of companies use GSM. Based on these similarities, one might think that most European cell phones work just fine in the U.S.

On the Other: The Frequency is Not

The reality, however, is that while the technology may be compatible, the frequencies on which those technologies operate is not. Europe's GSM phones operate on a 900 or 1800 megahertz frequency, whereas GSM technology in the U.S. operates on different bandwidths. T-mobile operates on 1900 megahertz, and Cingular operates on either 850 or 1900 megahertz, based on your location.

Bottom Line

Despite the fact that unlocked GSM-enabled phones from the U.S. can work in Europe as long as a new SIM card is installed, it does not work the other way around. In other words, while American GSM-enabled phones can work on the upgraded European frequency, European GSM-enabled phones cannot downgrade to the American frequency. There is always the option, however, of forgetting about all of this unlocking, GSM, SIM-card business, and just purchases an cell device that works on the frequencies of both continents. The plan for such a phone will be significantly more expensive, but such is often the price of mobile convenience.

Source:

"BargainOffers.com: The GSM Cell Phone Buyers Guide"

"PCMag.com: GSM Definition"