My Fi experiment is a solid A NB to A-. I use Fi as a unlimited voice/text plan for now.
There is no simple answer to your question because Fi favors mobile network for voice unless mobile signal is really poor. However here at home where both T-Mobile and Sprint Signal levels are modest to very weak, Fi does use my WiFi for calls sometimes but not always.
I have a couple of Fi Data-only SIMs but need to find a tablet or phone to use them with.