T-Mobile do not allow bar you from downloading Skype.
They do prohibit its use via
GPRS/
3G/HSPDA.
If they discover you're using it this way, they apparently reserve the
right to cancel any contract you may have with T-Mobile
You can of course use Skype via wifi (although maybe not T-Mobile wifi
hotspots?)
Another alternative for you is to use 07744 callthroughs to ring Japan
(comes out of inclusive mins).
Regards
Simon Clark
Business Telecoms