The countries of Europe use the dual-band frequencies of 900 to 1800 while America primarily uses 850 to 1900. When shopping for an unlocked GSM phone, you’ll want a tri-band 900/1800/1900 (or 850/1800/1900) or a quad-band 850-900-1800-1900 if you intend to use it in the U.S. as well as in Europe.