There are basically three methods by which you can travel internationally with GSM phones. All three require a phone that works on the band of the country you wish to visit. Let's take a brief look at how these work, along with the pros and cons of each.
1) Roam the world with your current GSM phone and SIM. Both Cingular and T-Mobile offer international roaming plans. This is by far the simplest option, especially if you will be on a cruise ship or plan to travel to several countries over a brief period of time. All you have to do is sign up with your carrier for international roaming. You keep your regular phone number so that relatives and friends can call you from back home. You can also send text messages and use data accounts. The only downside here is cost. Expect to pay from around one to three dollars a minute. Rates vary from country to country. Carrier websites break down the rates by country so you can plan phone costs as part of your travel budget.
2) Use your current phone, but buy a new SIM in your destination country. This is your best bet if you plan to spend a fair amount of time in one or two countries, or if you frequently travel to a particular foreign country. It is now easy and inexpensive to buy a SIM with a prepaid plan most any place you go. You will have several competing companies from which to choose, but all offer pay-as-you-go plans. Some pros here: You can buy more time at easily recognizable outlets found at shops, kiosks, train stations, etc. Rates are also much cheaper than standard international roaming (although still higher than your contract plan back home). The next time you reach your favorite international destination all you have to do is pop in your SIM and start talking. Cons include: You will have a new phone number. If you leave the country in which you purchased the SIM steep roaming charges will apply. And most importantly, you can only use a foreign SIM if your phone is unlocked! Currently, T-Mobile will provide codes to unlock most phones for international travel. Cingular, however, will not. (For more on unlocking GSM phones, see our guide, Understanding Unlocked Phones.)
3) Buy a cell phone with a SIM and pre-paid plan in the country of your destination. GSM phones have become cheap and easy to find nearly worldwide. For not too much more than a SIM itself, you can buy a cheap phone with a SIM already in it. Like method two, this probably only makes sense if you will be in one country for a long period of time. Pros: In addition to the others listed in method two, you don't have to worry about the phone being locked. You also don't have to worry about which bands it uses, as you know the phone will work in country where you bought it. Cons: You have to learn a new phone with new features. If you insist on a high-end phone this is not the way to go, since the really cool phones aren't usually available with pre-paid plans.