Cellular Network Technologies

The United States hosts two main families of cellular network technology.
Sprint and Verizon use CDMA and its faster progeny-1xRTT, EvDO, and EvDO Rev A-nationally;
meanwhile, AT&T and T-Mobile work with GSM/GPRS and a speedier upgrade, EDGE, nationally.
Of the two, GSM/GPRS is a little more flexible because all service and subscriber information is stored on a small, removable SIM (Subscriber Identity Module) card.
In theory, you can switch carriers simply by replacing one service's SIM card with another's; conversely, you should be able to switch handsets by slipping your current SIM card into a new phone.
In practice, however, a number of GSM carriers-including AT&T and T-Mobile--use software that locks a handset to a specific SIM card. To use a different SIM card, you must unlock your handset-a process that usually involves entering a code that you obtain from your carrier.
In some cases, a GSM carrier will help you unlock your phone if you have been a subscriber for a certain period of time. For instance, T-Mobile users can unlock their phone after 90 days, though the company says that an unlock code isn't available for all cell phones.
Some mom-and-pop cell phone shops and Web services will unlock certain GSM phones for a fee.
Another option is to buy an unlock code from sites such as Mobile Unlock, TravelInsider.com, and UnlockToTalk-or if you're lucky, you might find an unlock code on a forum by running a search for your particular model: "Nokia 5300 unlock code," say.
BlackBerry Curve 8300
$45 at Amazon.com with 2-year, $40/month voice and $30/month data plan.
• $100 at Newegg.com with 2-year, $60/month voice plan.
• $300 at AT&T with 2-year, $50/month data plan ($100 mail-in rebate available).
• $350 at eBay, unlocked version. Prices are as of 6/28/07
Where will you get the best deals?
I did some comparison shopping on the Web, at a few local retailers, and over the phone. The best place to start is with an online pricing engine, such as DealTime, Google Product Search, MSN Shopping, Pricegrabber.com (PC World's pricing engine), and Yahoo Shopping; use the engine to check out sellers and estimated prices for the phone models that you're interested in.
For the best deals and the widest selection, online stores generally beat brick-and-mortars and phone-order systems hands down.
Also, the carrier must have network coverage in your area
If something goes awry, you'll have to convince either your credit card company or the PayPal service to reverse the charge--an inconvenient hassle.
Carriers normally sell GSM handsets locked to keep you from switching services by swapping out the SIM card. But you can buy an unlocked phone from a third party.
The benefits: No contract is required, and you're free to use any SIM card at any time, including cards from overseas carriers. If you're a frequent overseas traveler, your cell phone needs will differ from those of homebodies. Here are key options, viewed from a globetrotter's perspective.
GSM vs. CDMA: In most cases, a GSM phone is the better choice because GSM networks are widely available throughout Asia, Europe, Oceania, and South America. But you should get a quad-band or so-called world phone--a handset that supports GSM on the 800-, 850-, 1800-, and 1900-MHz bands--because foreign nations don't use the same GSM frequencies as North America does.CDMA networks, used in the United States by Sprint Nextel and Verizon Wireless, are less common overseas, although they are widely employed in Japan and Korea. Sprint or Verizon customers who travel a lot should consider a CDMA-GSM hybrid (see "Dual-Mode Cell Phones," below, for more details).
International roaming: If you have a GSM world phone (or are using your CDMA phone in a region that supports it), you should be able to get it set up for international roaming with your carrier, but be prepared for voice and data charges that will be much higher than what you're accustomed to paying back home (check with your carrier about rates). When traveling, turn your phone off when you're not ready to answer it, or you'll pay even more for calls that aren't sent directly to voice mail.
Using an overseas carrier: Instead of roaming on a GSM world phone, consider opening an account with a local carrier at your overseas destination and swapping in that carrier's SIM card for your usual one. (You typically cannot activate a CDMA phone on a different carrier's network.) But you'll need to either buy an unlocked GSM phone or ask your carrier (or someone else) to help you unlock your handset. Our chart lists the pros and cons of this approach versus international roaming.