How Global Cell Phones Work

By: Dave Roos
SIM card
Swapping out the SIM card in your phone may let you use your own phone when you travel -- depending on where you go. See more pictures of cell phones.
© iStockphoto/oonal

Cell phones are an integral part of our lives. We use them to stay in constant contact with family, work and friends. We swap text messages (SMS) in traffic. We send tweets from the street. Heck, we even surf the Internet in the bathroom.

That's why our cell phones are so painful to give up when we travel to places where they don't work. In the past it could be very difficult -- or at least very expensive -- to use your cell phone outside of your home network. But the times are changing.


For years, cell phone carriers in the United States relied on technologies that were incompatible with those used in Europe, Asia and much of the rest of the world. Recently, two major U.S. cellular carriers -- AT&T and T-Mobile -- built nationwide networks based on Global System for Mobile communication (GSM) technology, the international standard for cell phone networks.

GSM has several advantages for international travelers and businesspeople. Since so many countries use the GSM standard (219 at last count), you can be fairly sure that your GSM phone will work no matter where your travels take you. That is, as long as your phone transmits on the right frequency -- more on that later [source: GSM World].

­Even better, an unlocked GSM phone allows you to switch out the Subscriber Identity Module (SIM) card. -- the internal chip that stores your phone number and account settings -- for a local phone number. Outside of the United States, most GSM phones come unlocked, but inside the country, you need ask your carrier for a special code. So if you know you're going to be spending the next four months in Botswana, you can spend $20 for a local SIM card instead of paying hundreds of dollars in international roaming fees.

There are also several companies that specialize in international cell phone service. They sell special global cell phones with SIM cards that use the same phone number and connect to any local network seamlessly for relatively low per-minute calling rates.

But before we get deeper into how to use these exciting cell phone technologies, let's look at the mechanics of global cell phones.


Mechanics of Global Cell Phones

Cell phone tower
If your cell phone isn't compatible with the frequencies coming from this tower, it doesn't matter what SIM card you're using.
© iStockphoto/jane

As you know from reading How Cell Phones Work, cell phones communicate via radio signals. Since the radio spectrum is a crowded place, cell phones are assigned certain frequencies at which they can broadcast.

Unfortunately, different countries assign different frequencies to their cellular networks. So before you decide to travel with your cell phone, you need to make sure that it will even work in your destination country.


For GSM phones, there are four main international frequencies:

  • In the United States, Canada and Latin America, most GSM networks operate over the 850 MHz and 1,900 MHz (also called 1.9GHz) frequencies.
  • In Europe, Asia, Africa and Australia, most countries use the 900 MHz and 1,800 MHz (or 1.8GHz) frequencies.

When you go to buy a cell phone, pay attention to which frequencies or bands it supports. Most GSM cell phones are dual-band phones that only work on the two most common frequencies for that country. But if you plan on traveling overseas, you'll want to invest in a tri-band or quad-band phone that is able to communicate over three or four different frequencies.

As cell phones become even more popular and more robust, cellular providers have begun to purchase more frequencies to support 3G and even 4G cell phone service. In some parts of Europe, cellular carriers are already using the 2,100MHz frequency for 3G networks. And in the United States, Verizon has purchased space on the 700MHz frequency to offer even faster Web browsing and data transfer speeds using a new 4G technology called Long Term Evolution (LTE) protocol [source: Reardon].

Every GSM phone is loaded with a SIM card. The SIM card holds your phone number, contact list and other account settings. SIM cards can be swapped in and out of any unlocked GSM phone. So if you travel, you can buy a local SIM card with a local phone number and use it with your GSM phone.

A new SIM card, however, won't change the frequency coverage of your phone. You still need to have a phone that communicates over the right frequency for your destination country.

Now that we understand how GSM frequencies and SIM cards work, let's talk about some of your options when using a GSM phone abroad.


Using Global Cell Phones

The basics of using a GSM cell phone are the same in any country. Most people pay for their calls using a pre-paid SIM card. You buy the card with a certain number of minutes and then you can refill it (or "top it off") whenever your minutes get low. You can even set up your account to automatically buy more airtime with a credit card each time your minutes dip below a certain level.

­If you own a tri-band or quad-band GSM phone, then you can travel with the confidence that your phone will work in most GSM networks around the world. This allows you to keep your home phone number as you travel. The downside is that you'll have to pay international roaming fees, which can add up quickly.


Another option we've mentioned is to purchase a local SIM card when you travel. International SIM cards run about $20 apiece. This is a good solution if you travel often and mostly to the same two or three destinations. It's also a good solution if you're going to be staying in a single international destination for longer than a month.

The advantage of a local SIM card is that you don't have to pay international roaming fees. The disadvantage is that you'll have a different phone number with each new SIM card that you buy. This could make it hard for people back home to get in touch with you. You'll also need to re-enter all of your contacts if they're not stored on your phone.

Several companies offer what they call international cell phones or global cell phones. These are a good option for frequent travelers who rarely stay in a single destination for a long time.

Global cell phones are usually tri-band or quad-band phones loaded with a special international SIM card. These international SIM cards offer the advantage of a single phone number without expensive international roaming charges. The rates you pay per minute depend on where you are and where you're calling. Some international SIM cards offer rates of 50 cents a minute where the same call would cost you $1.69 a minute in international roaming charges.

Even if you only travel internationally every couple of years, a global cell phone could be a good investment. There are companies selling global cell phones for as little as $40. For the cost of two SIM cards, you'll have a cell phone with a single lifetime phone number that you can load with minutes when you need them. When you don't need it, you can just put it in the drawer next to your passport and wait for the next trip.

For lots more information about cell phones and travel gadgets, call up the links on the next page.


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Related HowStuffWorks Articles


  • German, Kent. CNET. "SIM card explained." April 12, 2005. (Feb. 17, 2009)
  • GSM World. "GSM" (Feb. 17, 2009)
  • GSM World. "GSM Roaming." (Feb. 17, 2009)
  • "International Mobile Phone Rental Solution" (Feb. 17, 2009)
  • Reardon, Marguerite. CNET. "Verizon Expects 4G Launch Next Year." Feb. 18, 2009 (Feb. 23, 2009)
  • Rowell, David M. The Travel Insider. "Choosing the Best International Cell Phone Service for You." Dec. 3, 2008 (Feb. 17, 2009)
  • TigerTV. "What is GSM?" (Feb. 17, 2009)