Is it easy to transfer a US eSIM to an iPhone in Japan?

It is always best to buy a phone in the county in which it will be used most of the time.

If you buy in Japan, you would want to make sure that the carrier that you choose in Japan will not lock the phone to their service. If they do, a carrier in the U.S. will not be able to set up service on the phone.

There is also a good chance that a phone purchased in Japan will not have warranty coverage in the U.S. It will be up to the service center to decide whether or not they can service the phone under the warranty. For example, some parts for the phone purchased in Japan will not likely be available in the U.S., because the phones are not the same.

If you buy a phone in Japan be very careful to make sure it is not locked to a Japanese carrier.

The warranty will not be valid in the US. It may be impossible to have it repaired at all without physically taking it back to Japan.

It will be impossible to silence the camera shutter sound as it is illegal to do so on devices sold in Japan.

I'm sure that T-Mobile can handle a phone with a SIM card......as long as it is unlocked.

The real question will be whether an Apple service center in the U.S. will be able to handle warranty service on the phone.

A friend who purchased their phone in Mexico learned that the phone could not be serviced in the U.S. under the warranty, for example. Of course, hopefully the phone will not require service.

Hi, your iPhone most definitely has a warranty. You can read the details in the document ( Apple Worldwide Warranty) at the link below. There are a few exceptions that might or might not apply to you.

Apple Authorized Service Providers have a right to limit warranty service, as an independent business. However, any of the nearly 300 Apple Retail Stores in the US will welcome you and your iPhone.

https://store.apple.com/Catalog/Images/worldwidewarranty.html

You’re welcome! I’ve first hand experience with warranty repairs on Apple products purchased overseas. I’m a former Apple Store manager near an extremely well known tourist location in Florida. Never really an issue once limitations, if any, were explained. In fact most users were just thrilled we could repair or replace the device.

Our advice.....based on experience......was to buy a phone in the country in which it will be used most of the time.

But, it's your decision. Good luck.