Really need to drag you out one day my way to see the weirdness of it....
With my iPhone at my parents place (which roams to Vodafone) I can make a call using WiFi calling absolutely no problems. Agreed however regarding handover back to the cell network - really would love to see this resolved as I always fall into this trap at home.
VoLTE will see handover sorted but when will NZ see 4G calling @saltyNZ
That I can't say, I'm afraid.
Before Vodafone do :)