My Colleague had a similar problem where T-Mobile thought that he was in Brunei when in fact he had landed in Singapore after flying directly from Japan.

In Japan it recognized him in Japan and he was able to use DATA without incurring any charges, but in Singapore he was welcomed to Brunei and was warned of high charges for Voice/Text/DATA.

He called T-Mobile and the agent while nice (according to my colleague) and wanting to be helpful had truly no idea how to fix the issue and said that best that she could do was note the account and if charges occurred to call in when he (my colleague) got back to the USA and hopefully the agent would read the notes and credit the account for the charges.

The word "hopefully" scared my colleague and consequently he did not use his phone as much as he might have and stuck to a local SIM in his iPAD for most of his DATA needs.

When he checked his bill, he was not billed as he was in Brunei so as best I can tell is that T-Mobile is accidentally sending out the wrong message every once in a while?