If you haven't already check out the other thread on this issue :
For what it's worth ... See the comments I posted from my call to XM last week. Obviously, it does not do anything to correct the problem, but does give a lot of insight into XM's total disregard to acknowledge there is a problem or make any effort to fix it at this time.
Hey stormzx28, we are aware of call failed issues affecting some customers using WiFi calling. If you have a case or ticket number from XFINITY Mobile, I'd be happy to take a look into any updates from support. Feel free to send me a private message with your full name and the last 4 digits of your card on file so I can access your account and check. Just click my name and then on the next page, you'll see an option to send me a private message on the top right hand side.
Hello, I too am having constant call-failures for incoming calls since switching from AT&T to Xfinity. Restarting my phone will solve the problem, only temporarily. My phone is not set to make WiFi calls. It’s an iPhone 6 Plus....
I look forward to your help resolving this issue! Thanks!