Telephones have come a long way from the rotary dial of our parent’s generation. These days, your smartphone puts more computational power in the palm of your hand than the original “supercomputers” from the 1960s and 1970s did. All of that power comes with a price, however—and especially when you travel.

We recently showed you how to save money on international cell phone use. Now we’ll look more closely at one of those ways: how to save money by switching out your phone’s SIM card the next time you go overseas.

If you’re wondering what the heck a SIM card is, don’t worry—we’re going to cover that, too.

What is a SIM card?

A SIM (Subscriber Identity Module) card is a small plastic card that goes inside any phone that uses the Global System of Mobile Communications (GSM, for short). SIM cards also store basic information (name, number) about the user of the phone.

GSM is the system most frequently used worldwide among cell phone users; some American cellular providers like Sprint and Verizon use the other major cell phone radio system, Code Division Multiple Access (CDMA). Overall, a greater number of US-sold phones are compatible with GSM. Check with your provider or look for your network on this chart to see if your phone is SIM-card-compatible. You also need to find out if your phone is “unlocked,” which means you can switch between different SIM cards.

SIM Card Options for International Use

If your phone is locked, but uses the GSM system, you can still use it in foreign countries. Ask your mobile provider about international roaming plans. Sometimes you can purchase a fixed number of minutes, text messages, and/or data to use during your trip.

Another option for a locked or GSM-non-compatible phone is to use a cheap local phone and SIM card while you’re overseas. This could be especially helpful for longer trips, such as a semester studying abroad or a one-year English teaching gig, because it gives you affordable local communication (calls and texts) while you use a computer to message and talk with family and friends at home.

A local SIM card is one you purchase in the country you’re visiting. It gives you prepaid minutes and texts to use only in that country. As mentioned, this is a good option for someone planning an extended stay in one place, whether for work or leisure. For example, when I moved to Prague to teach English, I bought a basic cell phone and local SIM card from a Vodafone store. When I was ready to leave I gave the phone and SIM card to another teacher. So if you want to get a local SIM card, you may be able to find someone ready to pass one on or sell it at a discounted price.

An International SIM card can be used worldwide. It allows you to switch between an American number (even the one you already have)  and foreign numbers so the people you’re communicating with don’t have to pay extra to call or text you, regardless of where they are and what country you happen to be in at the moment. Incoming calls and texts are usually free, and you can choose from a variety of prepaid calling, SMS, and data plans.

An International SIM sticker is meant to be used with your current phone as long as it’s unlocked. Using a sticker instead of a different card is convenient because you only have to put it on once and then it activates whenever you travel.

A Europe-only SIM card may be a little cheaper than one that covers the entire world. It offers the same benefits as an international card, but is limited to European countries.

Best Deals on Global SIM Cards

Once you figure out which kind of SIM card best meets your needs, there are plenty of buying options. We’ve broken down the prices from the most well-known global SIM card companies so you can find the best deal. For each company we examine:

• Base charge for acquisition of the card: How much it costs and what’s included
• Data charges per MB: This may be the most important consideration, as most of us use our smartphones as much for apps and web browsing as for old-fashioned voice calls and texts.
• Any other charges and fees associated with the purchase, such as shipping and the cost of buying more data or minutes.

Telestial

• A $19 International SIM Card comes with a $10 credit you can use for talk, text, and data. You can add pre-paid credit through Telestial’s app or by phone.
• Telestial’s Europe SIM card is the same price.
• Friends and family can use a toll-free number to call you.
• Telestial charges the same rate per minute, text, or MB of data. Rates vary by country; for example, the flat rate is $0.25 in France but $0.59 in Japan.
• You can also buy 250 MB of data separately for $19. The price jumps to $29 if you purchase it after buying the SIM card.
• The SIM card comes with a +44 British Isles phone number. You can add a US number for $2/month.
• Shipping is free with an add-on credit or data plan purchase. Otherwise shipping costs start at $10 for US orders.

WorldSIM

• The International SIM Card itself is free. At checkout you add pay-as-you-go credit for $27, $40.50, $67.50, or $135. All of the options above $27 come with one to six months of free global wifi (accessible through the WorldSIM app and local hotspots).
• It comes with UK and American phone numbers (2 numbers total). You can add additional local numbers for $2.50 each.
• Rates per minute, text message, and MB of data are different and vary depending on the countries you make the call from and to. You can check rates on their website.

OneSimCard

• The International SIM Card costs $29.95 and includes two phone numbers from Europe and the US. Incoming calls to the European number are free, as are incoming text messages to either number.
• Free shipping on orders over $50
• This is a great option if you belong to a travel rewards program. You can earn one mile for every incoming call longer than a minute and outgoing calls of any length.
• OneSimCard’s rates on other services are a little harder to figure out than other companies. You have to choose your home and destination countries; then there are a la carte rates and discount packages you can purchase to receive lower rates on calls, texts and data for a specific period of time. Discount packages are separated by data, voice calls, and texts.
• The website features recommendations from many well-known magazines and newspapers, but the overall price seems higher to me than other brands. Whether it’s a value for you will depend on how much you wish to use your phone for calls, texts and data.

KnowRoaming

• Their Global SIM Sticker (as mentioned above) is $29.99 with $10 in credit included. Unlimited data starts at $7.99/day. Pay-as-you-go rates vary by country. In France, for example, data usage is $0.15/MB and outbound text messages to any country $0.14/message. Incoming texts are free. Phone calls are around $0.13/min depending on what type of phone (landline or mobile) you call and where.
• The Global SIM Card is $9.99. It doesn’t come with any prepaid credit. Pay-as-you-go rates and unlimited data packages are the same as for the sticker.
• KnowRoaming offers free use of WhatsApp wherever data is available.

What if my phone is locked or doesn’t work on GSM networks?

Most of the same companies that sell international SIM cards also sell and rent unlocked phones, so you could buy a separate travel phone if your American phone is locked or incompatible with GSM networks. You could also buy a cheap local phone upon your arrival in another country.

Summary

Now that you understand the range of options and prices, you can decide if an international SIM card will help you save money when you travel.

Infrequent travelers may find it cheaper or more convenient to simply add a temporary roaming plan to their existing mobile coverage. If you travel more often, or you’re planning an extended stay in one country or region, you can save money by using an international SIM card.

Pay-as-you-go lets you stay in control of how much you use your phone and what you pay for it. Just figure out how often, and for what purposes, you think you’ll use your phone. That should guide your choice of which SIM card and how much prepaid credit to buy.

Read more:

• Traveling Abroad? The Best Credit Cards for International Travel—And Some To Leave At Home
• Travel For Cheap: 6 Ways To See The World For Less