In my experience, WIFI calling is a hit&miss feature, even carriers don't have the answer. Carriers blame the handset, they say yeah your mother's works because it was originally a x-carrier branded, but yours is a y-carrier branded. Handset makers of course when you call them, they refer you back to the carrier. Some carriers, having running out of explanations, would tell you, if you buy OUR phone, that will work for sure. DUH!
I gave up, currently am using Google Hangout for VOIP calls, can't receive at the moment but I have not kept up with the latest since last spring, there are other variance like Magic Jack just another VOIP provider I hear, not endorsing.