AT&T and T-Mobile are the two main providers (in the USA) who provide GSM service. Below are the main cellular bands they use.
AT&T:
GSM: 850 and 1900
3G / UMTS / HSDPA: 850 and 1900
T-Mobile:
GSM 1900
3G/ UMTS / HSDPA: 1700 (Unique to T-Mobile) and 2100MHz
GSM bands not supported in the US: 900 and 1800
Internationally, you may need the 900 and 1800 bands, especially in Europe and Asia (many south American countries work on the 1900 GSM band). If you have a “quad-band” phone it will work everywhere.
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