At least I *think* that was the popup message.
I wanted to post this here to help others out if they should encounter the same issue.
So we recently switched to Xfinity Mobile and got new phones, iPhone 8 plus's to be exact. When I went to turn on wifi calling a popup said something like "contact Xfinity Mobile to enable wifi calling". So I did. And I spent about 20 minutes on the phone double checking with the tech what I had already done (made sure I had the latest iOS update and verified the carrier settings). His advice at the end? "Make an appointment with Apple". Hah! The popup clearly said to contact Xfinity.
If you run into this, it is my guess that restoring the phone from a prior backup somehow can cause this issue with the software. The solution is quite simple, restart the iPhone and you should be able to then enable the wifi calling and setup the 911 address. (which was not showing up for me before).
Is your cellular data turned on? I have the SE which I normally have data turned off but had to turn it on to complete the process of turning Wi-Fi calling on. Once completed I turned data back to off and Wi-Fi calling works for me. I am on by the gig is why I always have data off.