I used Fi going in the other direction today (call to an international number from the US) and it worked pretty well. The extra per minute charges (much more reasonable than my primary T-Mobile plan, BTW) are also already appearing in the Fi app.

The only thing is that the ringing tone sounded like the normal US one instead of the the one I expected (double ring due to the number being an Irish one). I'm not sure if this is normal or not, however.