But whether your carrier uses CDMA or GSM, you still need to unlock your phone--a software operation, not hardware--to swap in a different SIM (Subscriber Identification Module) card. A SIM is used to authenticate you to a cellular network and contains information that lets carriers sort out whom to bill.
Carriers in America are required to unlock phones promptly since the passage of a bill last year by a frustrated Congress--the House voted 69 percent in favor and the Senate was unanimous!--and rapidly signed by the president. However, with AT&T and T-Mobile, you need to have paid off the cost of the phone under contract or by paying termination fees. Apple provides general instructions on unlocking. You can visit unlocking pages at AT&T and T-Mobile to check requirements and complete the process. Verizon says it doesn't lock iPhones nor most other phones. Sprint has some requirements for unlocking with non-U.S. networks, but is generally amenable.
Once unlocked, you need to find a SIM that works in a country you're visiting. You can purchase SIMs from several firms that will deliver them to your U.S. address, and which you can swap in when you travel. Some come with a local phone number in the country you've visiting; others can be restricted for data-only use. For quick and infrequent travel, like ours to Canada, it's not a great deal, as the SIM costs $20 to $30, sometimes including credit.
For instance, Telestial will sell you a European-only SIM for $19 with $10 of credit or an international one (55 countries for data) for $29 with $20 of credit and both US and UK phone numbers. Data is $0.25 per MB ($250 per GB) for the European card and $0.35 per MB ($350 per GB) for the international one. They also offer a card preloaded with 1GB of usage that expires in 30 days after activation for $99.
You might also check OneSimCard, which has $0.25 per MB data, but also data packages between $25 for 250MB (used within 14 days) to $99 for 2GB (used within 30 days). Cellhire has various fixed-usage, no-overage data bundles, while the National Geographic Society (yes, them) has a branded SIM via CellularAbroad that bundles data, text, and calling in units of $29 with data about $0.50 per MB in most locations.
There are a lot of combinations, and I haven't yet tested any, so I suggest reading online reviews and asking colleagues and friends. You can also look into whether a carrier in the country you're visiting will sell a SIM and prepaid service, which many do; Three in Ireland has a €20 ($22) per month prepaid unlimited data usage plan, for instance. Again, this is more worthwhile for longer or repeat visits than one-off or short trips.
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