Your problem is odd, as although I don't even have a smart phone to use WiFi calling, I do in fact use a VOIP phone line running on a TP Link 4G router, or a EE mini hub for a duplicate VOIP line which works over my normal broadband. As it happened, today, we lost our Sky broadband for a short time, and the standby 4G line was very useful. Sorry I cant offer any help on you particular problem, but it seems very odd that your WiFi calling does not work. But could you not try port forwarding on the router for the mobiles, or maybe temporarily setting up a DMZ to a mobile just to see if it is a port issue???