I have an iPhone 7+ and it shows up in my Settings -> Phone area but when I go to turn it on, it says to contact Xfinity Mobile. This makes sense as I had to do the same thing to enable WiFi Calling on my iPhone 6 when I was with Verizon. I'm on the phone on hold with them right now. I think that most of their customer service folks haven't been trained on this as the person that I'm talking with is having difficulty with this. She thinks that I can do this myself but it appears that I can't. I have 11.2.2 or 11.2.5 installed.
I'd love to get this working tonight as I don't have Verizon voice service in my office.
We are former Verizon customers and are very dependent on WiFi calling as reception is dicey where we live. We switched both of our iphone 6's over to Xfinity Mobile about a month ago. It has all worked perfectly for us. Can't tell the difference between Verizon and Xfinity WiFi calling.