Major carriers Sprint and Verizon networks mainly use Code Division Multiple Access (CDMA) technology, while AT&T and T-Mobile use Global System for Mobile communication (GSM) technology. All of these carriers also support 3G networks. The network plays a big part in the capabilities your phone will have and, to some extent, its performance.
GSM phones have a SIM card that stores your account information and, if you choose, your phone book. When you switch to a new phone, you can simply remove the card from inside the old phone and insert it into the new one. But you can't use a T-Mobile SIM card in an AT&T phone, or vice versa. Also, GSM phones work more widely across the world than do CDMA models.
In our tests, CDMA phones generally have modestly better voice quality than GSM models. Historically, CDMA phones have had more useful features, such as a standard headset connector. Carriers also call the shots on features. For example, voice command—which lets you dial numbers from your phone book by speaking a name, without "training" the phone to recognize your voice—is almost standard on Sprint and Verizon phones but less common on AT&T and T-Mobile. Also, Wi-Fi is harder to find on smart phones from Verizon and Sprint than on AT&T and T-Mobile phones. Verizon offers phones that can switch to GSM when used outside of the United States.
Phones that support the fastest wireless broadband data networks, also known as "3G," are best for Web surfing, streaming videos or music, or downloading data-heavy attachments. Verizon and Sprint Nextel customers should look for models that support EV-DO data networks. AT&T and T-Mobile customers have a more-complex choice. The most common 3G protocol on these carriers is HSDPA, but there are also variations, such as HSUPA and HSPA, which have their own sets of data speeds and other characteristics. In any case, AT&T's and T-Mobile's 3G coverage are spotty compared with Verizon, and Sprint. 3G phones typically "downshift" to slower data protocols (1xRTT, EDGE) when 3G service is not available. Check with your carrier to see which broadband data networks are supported in your area.
You may encounter phones described as quad- or tri-band, dual-band, or multi-network. Those terms describe the ways a phone can connect to one or more wireless networks, which affects the coverage your phone provides. Here are the specifics: