I have lived between the US and Japan for the past 11 years. I have a Japanese iPhone and just switch SIM cards when entering a new country. I paid my carrier to unlock it before my trip to the states the first time. The only difference in the Japanese iPhone is it makes a shutter click sound when you take a photo and makes a chime sound when you start a video.
The law here requires it. If the phone is a recent model and unlocked you should be able to get a SIM card from a US carrier to use it. I’d recommend checking with the carrier first.
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