I've had good luck with T-Mobile's free international data service in many countries, but have had trouble in Japan. In Japan my experience is that roaming cell service will work for the first few days, and then silently cease working (almost as if there is a silent max data usage cap for T-Mobile users roaming in Japan).
In the case of Japan, depending on the length of the trip and phone use needs, OP may wish to investigate purchasing a Japanese SIM card and skip T-Mobile's free international data service, which OP isn't eligible for anyway due to having a prepaid T-Mobile plan.
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).