I tried entering my previous address with Xfinity into the WiFi calling 911 page and it worked. I now have WiFi calling on my iPhone after a frustrating week trying everything else that an Xfinity tech suggested. Now I have to figure how and why this ancient address still lives in my Xfinity file, but that’s a fix for another day.
Had the same issue. Xfinity and Apple support no help Here's what fixed the issue for me (iPhone X BYOD coming from AT&T):
Under Settings, go to Wi-Fi, then under Ask to Join Networks, select Notify (other choices are Off and Ask). Then go to Cellular, xfinity mobile, make sure Wi-Fi Calling is set to On. At this point, you should be able to enter your emergency address. Once you enter and confirm, my Wi-Fi calling remained set to on (previously it reverted to Off). I think the trick (for me) was making sure the Join Networks setting under Wi-Fi was set to Notify (and not off). Since doing this my Wi-Fi calling has remained on with no issues.
BTW, thanks for nothing xFinity and Apple support.
Ask to Join Networks
Changing these 2 settings allowed me to enter my emergency address (after turning Wi-Fi Calling On) and the Wi-Calling remained on (instead of reverting back to off).