If you're using data, the charges wouldn't be billed as minutes. If you're looking for those rates, you'd want to check out the coverage map on our International Plans | Traveling Abroad without Roaming Fees | T-Mobile and search the country you're going to. That'll give you the charge/minute and give you info about data usage based on your plan while you're roaming.
It sounds like you are making an intl. call FROM the U.S. If so, then the data services like Skype, Whatsapp, Google Voice etc. just take from data. No TM voice minutes.