The "3G" denotation in new phone indicates a "3rd generation" of wireless frequency abilities. A finite amount of wireless frequencies exist, which are leased by government to cell phone carriers for telephone and data use. Many factors will determine which of these frequencies a 3G phone uses.
Country of origin
The most important factor determining which 3G frequencies will be used is the country for which the phone was built. An iPhone purchase in and for the U.S. will operate in the 850 and 1900 MHz spectrums; the European iPhone 4, by contrast, operates on the 900, 1800 and 2100 MHz spectrums.
Carrier
Some carriers operate all over the world and thus many emphasize that their phones support frequencies that will meet those markets. T-Mobile, for example, is widely available in Europe as well as the U.S., and thus utilizes the 1700 MHz "AWS" as well as the 900 "GSM" spectrum. Phones made specifically for AT&T will likely support 700 MHz, as this company has purchased large amounts of frequencies in that spectrum.
Phone Manufacturer
Manufacturers of 3G phones often enter into deals with individual cellular companies. The iPhone, for example, was initially built exclusively for the AT&T network. AT&T updated its network in order to accommodate some of the more advanced feature requested by Apple, which makes the iPhone.
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