A Subscriber Identity Module card contains a microchip that phones compatible with Global System for Mobile Communications use to identify users on a cellular network. SIM cards encode service plan and account information, encrypt transmissions, and can store text messages and basic contact information, such as names and telephone numbers. Most SIM cards purchased from United States cellular carriers are limited to a specific network and will not work on competing networks, especially for locked phones. However, network-independent, third-party SIM cards are available. These can be useful options for those who cannot commit to long-term contracts and for travelers looking for cost-conscious alternatives to expensive, overseas roaming charges.
All GSM-compatible phones require a SIM card to function. Network-specific SIM cards are sold with a cell phone when customers sign a two-year contract. In addition, newer phone models like the iPhone 5 and new iPad use the micro SIM format versus standard-sized cards and some SIM cards work only on specific frequency bands. T-Mobile micro SIM cards, for example, require phones that operate at the 1700 and 2100MHz frequencies; AT&T requires its 4G devices to operate at the 700, 1700 or 2100MHz frequencies and 850 or 1900MHz for its 3G devices. Finally, users wishing to avoid long-term contracts can purchase prepaid SIM cards separately; however, these cards will only work with unlocked phones.
Understand the Plan Pricing Structure
A SIM card's price depends on its features. Most SIM cards include basic phone services, while some offer phone service only with the additional purchase of a data plan. Most offer both data and phone, although carriers may charge a higher rate per MB for data when you do not specify the purpose upfront at the time of purchase. Some carriers offer reduced rates for 1GB, 5GB or unlimited data access plans, either daily or monthly. The AT&T GoPhone prepaid SIM card, for example, offers basic calling services for unlocked GSM phones but provide data options only with its $65 monthly plan (links in Resources). Also, international SIM cards require direct activation at a carrier store, not a hotel or store kiosk. Ask questions and understand the terms of the contract, coverage area and how long the card will be valid or active before making any SIM card purchase.
Make sure your phone works on the GSM network of the country you are visiting. Otherwise, you may have to purchase a fully unlocked phone before you leave. When traveling overseas, prepaid SIM cards are the only option available for cell phone usage, so knowing how to contact customer service for assistance in English, activating and deactivating cards and refilling the card is extremely important. Some countries accept in-country credit cards or prepaid vouchers only for card refills. Also check with carriers to see if SIM cards support voice-over-Internet calling. Purchasing a local carrier card in each country you visit might be a more prudent economic decision than purchasing one card for to cover all your travel destinations.
Know How to Configure Phone Settings
Some GSM-compatible phones can auto-configure the SIM card for their unique settings, while others require manual configuration. If your phone is not configured for the SIM card, your phone will not operate properly. When purchasing a third-party SIM card, ask the representative for assistance in installing and configuring your phone to use the card.
Other Useful Tidbits
Carriers usually require users to activate new SIM cards, either using the phone's IMEI number (AT&T) or other number printed on the card. Plus, understand that SIM cards can contain multiple phone numbers.
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