Some people think that bringing a cell phone on a trip to another country is excessive. A part of me used to think that, too--until I tried it. One test drive and I was hooked. Not only is it great for emergencies, but it also comes in handy for sending quick text messages back home. If you're a spontaneous traveler like me, you can book hotels, dinner reservations, and the like as you go. If you have a camera phone, you can send snapshots of your vacation without visiting a photo lab or an Internet café.

But toting a cell phone on a global trek or a business trip requires a particular type of phone and wireless service--plus extra cash for the roaming fees. Not all handsets work outside the United States, and not all wireless service providers offer international roaming: In some cases you must get a new phone and sign up with a different wireless carrier. Here's a rundown on the things you need to know.

The Essentials

To use a cell phone overseas, you generally need a handset that supports Global System for Mobile networks in foreign countries and either an international roaming plan from your U.S. carrier or a local Subscriber Identity Module card from a telecom company in the country you're visiting.

Even if your U.S. carrier has a GSM network (AT&T/Cingular and T-Mobile do), your mobile phone isn't necessarily a world phone; if it's a relatively inexpensive handset, it probably isn't. And GSM comes in many flavors: Some phones support all GSM frequencies, and others are compatible only with one or two types. For a true world phone, look for a phone that supports GSM 850/900/1800/1900 (each of these numbers represents a different GSM frequency).

The U.S. and Canada, and parts of Africa, Central America, and South America, use GSM 1900. Asia, Australia, and Europe, and parts of Africa and South America, operate on both GSM 900 and GSM 1800. Some parts of Latin America also use GSM 850. Head to GSM World for a helpful list that shows which GSM frequencies are supported in nearly every country in the world. The site also provides information on the telecom companies available in each country, plus each carrier's network coverage map. It's a good idea to find out whether your phone supports the GSM frequency for your destination before you hop on the plane.

Motorola, Nokia, Siemens, and Sony Ericsson offer a variety of handsets that support all four GSM frequencies. For example, you can get the Motorola Razr V3 and the Nokia 3650 or the 6230 (which I used in Spain and Australia, respectively). LG Electronics, NEC, Panasonic, Samsung, and Sharp also offer GSM phones. A few PDA/phone combos such as the PalmOne Treos (GSM models) and Hewlett-Packard IPaqs work in other countries; RIM Blackberries do, also.

GSM handles voice calls. On the data side, the complementary services are General Packet Radio Service, the faster Enhanced Data for Global Evolution, and several so-called 3G (for third generation, the fastest wireless communications) technologies, of which only Universal Mobile Telephone System has begun to appear in the U.S.

International Roaming Service

To use a mobile phone abroad, you'll need to subscribe to an international phone plan. Such a plan allows you to call people overseas and receive calls from abroad while you're in the U.S.--albeit at a very high per-minute price. Text messaging your buddy in Calcutta is more economical.

The major nationwide carriers--AT&T Wireless, Cingular Wireless, Nextel, Sprint PCS,T-Mobile and Verizon Wireless--offer international roaming service. This means the companies have partnered with carriers in other countries to provide expanded service to U.S. customers. Some regional cellular companies also offer international roaming.

AT&T/Cingular and T-Mobile run on GSM, and Nextel runs on Integrated Digital Enhanced Network. Sprint and Verizon use Code Division Multiple Access. These carriers offer mobile phones that work in Asia, Europe, or other continents, although some handsets operate only in a few countries in a given continent. As with any cell phone service, you may not get coverage in remote areas.

If you're a Sprint or Verizon customer, be aware that your CDMA phone won't work on a GSM network here or abroad; you'll likely need another phone that supports GSM or both CDMA and GSM. Both Sprint and Verizon offer at least one hybrid phone for their globe-trotting customers. For example, Sprint has just announced its first quad-band device, the PCS IP-A790 by Samsung, which supports both CDMA and GSM networks.

Verizon notes that CDMA networks are available in some countries, including the Dominican Republic, Israel, Mexico, Puerto Rico, South Korea, and Venezuela. Verizon customers can use their CDMA phone in these countries, but they should check with Verizon before they travel to make sure overseas service is activated.

Nextel customers can use their Internet-enabled IDEN phones in a handful of countries with IDEN networks: Argentina, Brazil, Canada, Israel, Jordan, Mexico, Peru, the Philippines, and Singapore. Otherwise, they must have a IDEN-GSM hybrid (such as the Motorola T720) in order to roam on GSM networks abroad.

Alternatives to Roaming

If you'd rather not subscribe to international roaming with your existing phone, consider renting an international phone in the U.S. or at your destination. For example, Nextel, T-Mobile, and Verizon rent out phones to occasional travelers. An international phone rental can be pricey, however.

The less expensive option to international roaming is buying a prepaid SIM card at your destination, then swapping it with the card inside your phone. The tiny SIM card is like the phone's life-support system: Without it, the handset won't work. The card, which looks likes a circuit board, stores subscriber details, security information, and memory for phone numbers. The SIM card also stores data that identifies the caller to the service provider. The card is usually located one layer beneath the battery. Pop the battery out and you should see the SIM card seated in a slot.

Prepaid SIM cards typically come in per-minute increments. Costs vary, so shop around. Depending on the country you visit, you should be able to find a few phone operators selling prepaid SIM cards. In most locales, you'll find the cards at convenience stores and phone shops. If you want to control your usage, a prepaid SIM card is a good choice.

However, if you're planning on swapping out a SIM card overseas, you may have to do a little work before you go. Major carriers often make swapping out SIM cards difficult by locking their handsets--meaning the phone won't recognize cards from other carriers. In most cases you should be able to get the carrier to unlock your phone on request, but you should look into this before you travel.

Also, bear in mind that if you use an overseas carrier's SIM card, your phone will no longer answer calls to your usual number--that's one advantage to paying the higher rates for international roaming with your original SIM card.

Costs and Constraints

Like many monthly services, international roaming fees vary; and if you don't keep track of your usage, the bill can deplete your savings. For international voice calls, you're almost always charged according to the length of the call; prices range from 69 cents to a whopping $5 per minute. For text messaging, you'll pay about 10 cents to 35 cents per message. For Internet data, most carriers offer unlimited usage for a flat fee; Cingular charges $25, for example. Some also offer a metered usage option; for example, Verizon charges $15 for 5MB of data transmitted and/or received. On top of usage fees, you'll most likely pay tax and other surcharges--expenses that are difficult to budget for.

Each wireless service provider also has its own set of rules and fees. Some of the prerequisites are stringent and don't make much sense. For example, to get international roaming with AT&T Wireless or Cingular Wireless, you must be a subscriber for at least 30 days--forget about walking into an AT&T/Cingular store, buying a world phone, and expecting to use it in another country the next day. It's a good idea to call the carrier or check its Web site for details on international service months in advance.

Useful Extras

Although these accessories are an added expense, they're worth every penny:

Power Adapter/Converter Kit. This is one of those things that you need to bring on an overseas trip but almost always forget about until you need it. Before you go, I highly recommend buying a complete kit, such as Belkin's Business Traveler Converter Kit, which has both converters and tips for several major cell phones and PDAs. It's an indispensable tool.

Spare Battery. Finding that your cell phone has died the minute you get off the plane can be a real letdown. If you don't buy a power adapter, splurging on an extra battery is the next best thing.

Got a question or comment? Write to Grace Aquino.