SIM Cards

All GSM phones and some CDMA phones contain a removable SIM card (aka Subscriber Identity Module). SIM cards contain a unique account number that links the phone you’re using to a specific wireless network. In theory, all SIM cards can be removed from one cell phone and inserted into another. Unfortunately, many Smartphones are “locked” which means you can’t jump networks by swapping out SIM cards. Cell phone companies “lock” their phones to discourage SIM card swapping because the behavior tends to destroy the rational for subsidizing expensive handsets.

Jumping Networks

If you plan to swap out your US SIM card for a foreign one as you travel, make sure you have an unlocked phone. Unlocked phones are readily available anywhere in the US. Once you have an unlocked phone, you’re free to insert a SIM card obtained from any source. Be aware that if you use a “foreign” SIM card in your US phone, all your calls will be local to the country where you bought the card. You won’t receive calls made to your US phone number and you’ll be incurring hefty roaming charges if you call back to the US from your “foreign” phone.