I disagree, it's a great tool - T-Mobile has roaming agreements with Softbank, NTT and DoCoMo, not KDDI. The biggest issue is the phone bands, if your phone do

I disagree, it's a great tool - T-Mobile has roaming agreements with Softbank, NTT and DoCoMo, not KDDI. The biggest issue is the phone bands, if your phone does not support the local bands, it will not connect. I had no issue having a secondary phone in Japan w/ my T-mobile sim and having a local NTT sim. With dual sim/esim even easier nowadays to run both networks.