I have not been able to receive calls on my Xfinity Mobile. So hit and miss. As soon as I try to receive a call, Call drops with a message “Call Failed.” I have the iPhone XS MAX through Xfinity mobile. Called support a few back, they had me reset my Network settings. Seemingly helped for few days ( less than a week). I am back with the same problem. Outbound call work. Incoming call fail. What’s going on?
Try to disable the Wifi calling on your iPhone and check it out. I had this problem earlier but no longer. Initially when I bought the phone last May, I turned on the Wifi calling but had this frequent "call failed" issue but couldnt relate these two. But later on, on a whim, disabled the wifi calling and after that magically the "call failed" thing disappeared after few days. Try it out, and monitor for few weeks.