1 of 1 people found this helpful
"Pay in Advance (Prepaid) while traveling
Our Pay in Advance plans let you stay in touch when traveling abroad. Find out more by visiting the Pay in Advance international services & ratespage. Keep in mind, international data roaming is not available for Pay in Advance accounts." International roaming services
The link to the prepaid international rate page leads you off to the general prepaid page.
I think it's safe to assume that T-Mobile really isn't interested in supporting prepaid international roaming.
What phone are you using? It might help to get it unlocked before you return to Japan.
Just bouncing in for another check on the device before we close this out! Fingers crossed everything is just worked.