When I was in the states using Fido Wi-fi Calling, it seems after I turned on Wi-fi Calling, it won't switch back to LTE network if there's no Wifi, it only shows no service on the phone. And this never happened in Canada. My iphone 6 plus always very smoothly switch between wi-fi calling and LTE. Maybe anyone had the same problem when roaming in the states?
It will only switch to LTE if you are in a LTE area if you were in the states it would mean you were not in a Fido LTE zone thats why it did not switch to LTE when you were in the states.
You can see Fido coverage area here.