I believe the deal when roaming outside the US is as follows (same for Cingular):
If you turn on your phone and it registers on the other network, calls to your phone number get routed to your phone in whatever country you're in. If you answer the call, you are charged for roaming. If you don't answer the call, the call get routed back to your voicemail in the USA and you again get charged for a long distance call and roaming, so two charges for one call.
A good place to check is www.howardforums.com
in the T-Mobile USA section. Not sure if it will do you any good at this point.