This website shows which providers support which wireless GSM frequencies.
Useful(?) when you're looking for a GSM cell phone that you can use in different
parts of North America and the world. The phone has to use the frequency of
the provider, and providers must support the same frequencies so the phone
can use their network when in their country. Simple, right?
For example,if I wanted to use one phone in Canada and the States,
I figured out that I'll need a phone that can use the 850/1900 frequency
because both Rogers and AT&T support GSM phones (phones that use
SIM cards) that can access/use those frequencies.