Just posting in case this helps others..

Couldn't get iphone wifi calling working on my Deco Mesh.. tried all sorts.

Went for the port forwarding solution and a dedicated IP for my wife's phone.. then something odd happened as I was seting it all up... as I was setting the port forwarding for port 500, went to save it.. it complained that the VPN Server was already using that port ! - so I switched off all the VPN server (had all 3 protocols defined), configured port forwarding for 500 and 4500 and all worked well. Then I deleted the port forwarding as wondered if it was just the VPN Server that was messing this all up... and lo and behold.. Wifi calling works again, no port forwarding and no VPN Server..

Didn't think the various VPN servers went anywhere near those ports ?!

Hope that helps someone.