Another data point is you will get charged calling a non-US number via a WiFi call even if you have an international calling plan. For instance, I was charged 20¢/minute for calling an Australian landline in Australia even though I have an international calling plan and was placing the call over WiFi.

When reading the fine print, the international calling plan is only effective from T-Mobile's US network. I could test sometime to see if I'm charged when calling internationally using my WiFi connection within the US. It's strange because I would think a WiFi call from anywhere would be equivalent to calling from the T-Mobile US network.