I’m visiting the U.S. for 5 weeks and have a normal, KDDI AU iPhone 6 Plus. When I arrived in the U.S. I noticed the carrier signal said AT&T. So out of curiosity I went to an AT&T shop this morning and they said as long as it shows AT&T you can use it with a GoPhone prepaid contract.
So they took out my AU SIM and put in an AT&T SIM and sure enough it works as is! No unlocking necessary!
I got the $45/month unlimited talk and text + 1.5 GB plan.
I’m happy that I could use my Japan iPhone in the U.S. so easily and so just wanted to let others know who might be in a similar situation.
When I turned my phone on, by the way, I also got a text message in Japanese from AU welcoming me to the U.S. and telling me that for a flat rate of ¥2,980/day (expensive!) I could use my iPhone 6 Plus as is. Checking online, it appears you need to turn on roaming for that, which I did not do.
It seems much better to just get an AT&T domestic card for your trip.