I have an iPhone 6s Plus but I can't get the wifi calling to work. I've turned it on through my settings and I've had it activated through Vodafone. I've got iMessage turned on and my number is showing too. It worked for about an hour but it hasn't worked again since!
On the iPhone hold down the Sleep/Wake Button and Home button together until the Apple Logo shows. Then On seeing this Apple Logo release both buttons ,and the iPhone will reset , which has been known to clear most glitches.
Try turning Wifi assist off as this iOS 9 feature will automatically move your phone from wifi to network data if the iPhone thinks the signal is too low.
Settings-Mobile Data-Scroll down to the bottom to the setting.
If no help then perhaps speak to customer services via Live Chat or 191.
Power your router off/on too, I used to work for a well known router manufacturer and loads of manufacturers had the same issues with UPnP failing after a time... Not sure if Wi-Fi Calling uses UPnP or not but I had a similar issue on EE Wi-Fi calling... Worked on the neighbours Wi-Fi who has slower broadband than me and the signal was weaker, but not on mine. Quick power cycle on the router fixed it.
I'll try turning the router off and on and see how that goes. I've got a BT homehub 5, I've logged into admin but can't find the setting you've mentioned.
