If you use your T-Mobile sim and are paying for Internation, connecting to WiFi should work. If you are going to use a sim from a local service provider in Germany then it will not work as it is a T-Mobile USA only feature.
I'm planing on using my TMOUS SIM card. Would I have to pay anything extra to do this? I'm thinking not, as it is a WiFi call.
If you don't get the International service you may get hit hard on Data and Voice. If you use WiFi calling it can use your minutes unless you have it setup with T-Mobile to be free. You may want to talk to T-Mobile to make sure about charges you may get.