Someone briefly mentioned to me that U.S. T-Mobile customers can use their phones in Japan. How does this work?

My wife has an iPhone 7 with a T-Mobile Simply Prepaid plan.

Does it use WiFi? Is there anything she needs to do to activate service in Japan?

It doesn't seem to be available with pre-paid plans. You need to have a plan where they bill you monthly. I have the Simple Choice plan and I get data (slow) and unlimited texting in most countries, and a cheap calling rate. I remember switching from pre paid to that plan in order to get the international calling plans. You might be able to add international services, but you would have to check the T Mobile website for your plan.

If you have a plan that supports their free international roaming, you don't need to do anything. Just turn on the phone and you're set.

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).

We have Simple Choice and it just works seamlessly. This is not, however, a prepaid plan. I just fire up the phone when I land and it works. No added charge except for phone calls at a modest charge. Magic.

Try disabling LTE. This sometimes stabilizes my phone when I'm having roaming issues with T-Mobile. I've only had that issue in HK but YMMV.

Great suggestion - I'll sure try that on my next trip.

I had no problem using my T-Mobile phone in Japan, but yeah, as noted, it's not an option for prepaid plans.

Firstly, thanks to all. [MENTION=243397]mnbp[/MENTION], I'm passing this info on to my wife so she can give it a test run. Brilliant if it works in Japan. Thanks again!

Another option would be to have prepaid service with Project Fi. You will get full speed free data roaming at $10/gb in most of Asia. It works in Japan.

As everyone mentioned above... not for Pre-paid! I live all year long in Japan and my T-mobile is 24/7 Internet is slow, but it does the job and I can do everything I need from my mobile phone.

If you dont have the international plan activated, you will have to do this before you travel.

And the link below is the official Flyertalk T-mobile post... T Mobile Global data coverage

I hope this info is not too late, as you did not mention when was the trip!

It's weird that the service is slow since I believe T-mobile's Japan partner is NTT DoCoMO, which has the biggest network of all. Must be being intentionally throttled.

I've had T-Mobile monthly plan for three years now. I've used it all over Europe, Central America and Japan. Works great, but it is throttled to 3G, sometimes 2G, but not all the time. Sometimes LTE does work.

I've never figured out the pattern.

Maybe I'm old fashioned, but I never call home when I'm traveling in Japan, and if I have to make domestic calls, I just wait till I'm at my hotel.

I've heard that wi-fi is more available than it was on my last trip, four years ago, but I registered for Starbuck's free wi-fi and have been able to use it easily. The only trick is that you can't register IN a Starbuck's outlet, only from another computer. But that has been sufficient for my web browing and e-mail needs.

Our Sprint plan has free international roaming. Seems to work ok except in Jpn. Think it's thru Softbank, mostly 3G. Interesting to hear T-Mobile users complaining about similar issues in Jpn. What's the deal?

I believe Sprint has an option where you can get LTE coverage in Jpn for something like $20-30/week. Does T-Mobile have something similar?