What T-mobile is saying could probably be true. You may wanna see an important message in hello977.com website. It says

IMPORTANT: When
calling from a cell phone using a phone card, make sure that you DO NOT
press "Send" or "Talk" key after entering your destination phone
number. This prevents you from unintentionally using the international
call services provided by your cell phone carrier. We STRONGLY
recommend our customers to cancel international service with your
cellular phone carrier before placing calls through calling card
service.

The above mentioned things may have happened to you.