At the moment I travel with a factory unlocked i Phone 7 I bought without a contract (which means you have to pay full price for the phone, normally phones are subsidized by AT&T or Verizon).
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One option is to buy a separate cheap unlocked international phone to use specifically when you’re traveling.
Another option is to call AT&T or Verizon and ask them what steps you need to take to your phone, which may require paying a fee.
Once your phone is unlocked, you can use sim cards & data plans from other companies when you’re traveling. Data speeds are often throttled after a certain amount has been used, or coverage is sparse.
A whole new industry has sprung up around using your cell phone overseas, due to the high rates many mobile carriers like AT&T or Verizon charge for international roaming while traveling. In my experience, global sim cards and international wifi hotspots generally aren’t worth the cost unless you’re a serious business traveler with cash to burn.