Make certain you have the Xfinity profile installed on your phone.
The instructions don't indicate this is required for WiFi calling. The profile will help you connect to Xfinity hot spots and may help with WiFi calling. Regardless the profile needs to be installed.
I have - soon to be had - all services (phone, internet, TV and mobile) with Xfinity (and no problem until Xfinity Mobile) - I have called multiple time and the knowledge or ability to help me resolve my inability to set up WiFi calling does not exist - three new phones and none of them can be connected to WiFi calling, and none of them are recogniable as a mobile hotspot - and again the Xfinity Help Desk is no help - warning issues remain unresolved - and no returned calls from XfinityLevel 2 upport.
this is to bad, because if the phones worked, they would be a fantanstic deal - but a non-functional phone that the help desk cant fix is not so good a deal.