[John Clark]

That is totally incorrect. Either you misunderstood or he didn't explain it correctly.

If you DO have the H@H plan then calls over wifi are unlimited.

if you DON'T have the H@H plan then calls over wifi come out of your bucket of minutes.

It's as simple as that.

No matter where you are in the world if you connect to UMA then it's as if your phone is in the US on a TMobile tower. If you call to a foreign country you will be charged as an international call. If you call to the US (from a foreign country but on UMA) then it's a regular call and goes against your minutes (or unlimited if you have H@H.)