I am having the same problem. I've worked through the issue and have found the problem..BUT NO SOLUTION. When calling on WIFI the phone will show XM WIFI. Calls go through with no issues. When the phone shows XFINITY MOBILE no WIFI calls will go through. The problem is that the phone will switch randomly between the two settings.

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Wi-Fi Calling from AT&T You can now place and receive Android phone calls on your The Windows 10 Your Phone app can use your Android phone to make and receive calls. Microsoft With everything set up, you can now start handling your Android phone duties on your PC using Your Phone. 20 Best Free Internet Phone Calls Apps - Lifewire Google Voice. Runs on computers and mobile devices. Can forward all calls to your existing phone. … Five great Android Wi-Fi calling apps - CNET