Primer: Intro To Cellular Networks
Most national and even regional carriers in the US use more than one band; they have purchased the rights to spectrum in both the 850 and 1900 MHz range. Sometimes they use 850 in one area and 1900 in another. Sometimes they use both bands in the same area. In fact if you change cells during a phone call, you might even start the call on one band and end on the other.
When the FCC licenses a band of spectrum, they subdivides it into a number of blocks of varying sizes and auction them off. However winning this auction does not entitle a carrier to that block of spectrum nationwide, instead this process is repeated for every geographic area. A larger carrier may not win an auction for a single block large enough to meet its needs in some area. In this case the carrier may try to acquire additional blocks on the other band if they cannot purchase more space on the same band.
Some carriers, like T-Mobile, have been able to only use only one band (1900 MHz in their case) and will share spectrum with other carriers rather than resort to purchasing spectrum on the other band. However many of the phones T-Mobile sells work on additional bands, often including support for 850 MHz or European bands.
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