Try this
https://play.google.com/store/apps/details?id=com.wifi.calling
My phone was the same but this let me force change a hidden setting to prefer WiFi calling and it seems to have worked since
I successfully activated a SIM card from SMARTY in France once, SMARTY roam on Orange, Bouygues, Free and SFR over in France.
This was using a UK debit card, but I can't see any reason why you couldn't use a French one!