Pick a cell phone service provider
Wireless carriers in the United States operate over two different networks: Code Division Multiple Access (CDMA) and Global System for Mobile Communication (GSM). Though each technology transmits voice and data, they do so in different ways, which makes them incompatible. As a result, you can't take a CDMA phone and use it on GSM or vice versa.
Of the U.S. carriers, AT&T and T-Mobile use GSM while Sprint, Verizon, and smaller carriers such as MetroPCS and U.S. Cellular use CDMA. Though Nextel is part of Sprint, Nextel-branded phones use a third technology called iDEN, or Integrated Digital Enhanced Network. Many of these carriers offer prepaid services. Verizon, for example, has its own prepaid plan page.
CDMA coverage is very strong in the United States, particularly in rural areas, but GSM service has a larger global footprint (it's the standard in Europe, for example) and GSM phones use the convenient SIM cards, which allow you to, among other things, switch phones more easily. Also, when taken on a global scale, GSM users will find a wider selection of handsets. Note: SIM cards now come in a smaller Micro-SIM form factor to accommodate thinner handsets. Not all phones support both sizes, so be sure to check and see what size cards the phone uses before swapping handsets.
With that in mind, if you travel overseas frequently or you enjoy switching out your phone often for the newest model available, then GSM is the better choice. Not all GSM phones will work overseas, however, so be sure to read CNET's quick guide to world phones. But if you'll be making calls mostly in the United States, then CDMA is an equally good option. What's more, some CDMA phones now also support GSM networks for international use.
The Nokia Lumia 800 uses GSM.
