Originally Posted by mnbp

Or, bypass Japan cell carriers entirely, leave the phone in airplane mode in Japan, and just turn on the phone's WiFi and use that instead. According to this, all recent iPhones support T-Mobile's WiFi calling. Check it out at home first, so you know how to do it in Japan. (turn on airplane mode, then turn on wifi, then see if you can make a call over wifi - just my guess as to the procedure, I don't have an iPhone myself).