This has been a long-standing problem mainly with iPhones. I experienced this with my iPhone XR and finally just shut off Wi-Fi calling. FYI after doing this it was necessary to turn off / reboot the phone to completely clear the issue so incomming calls wouldn't immediately disconnect with the "call failed" message.
The problem seems to have been fixed with Apples's iOS 13.4 update. Or maybe Xfinity's recent carrier settings update. Make sure you update and also have Xfinity carrier settings 41.0. or later (Settings -> General -> About -> Carrier).
I have not had any problems since. Other users have confirmed this. My phone stays locked on XM Wi-Fi where before the connection was random. No disconnects so far. In the past I had also "Reset Network Settings", reconnected to my Wi-Fi and turned off "Ask to Join Networks". I don't want the phone automatically connecting to Xfinity hotspots as many are "not connected to the internet" for whatever reason and when traveling the phone would connect to the dead hotspot and lose internet connectivity.
Finally !!! - Fingers crossed.
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Previously my iPhone XR would randomly connect to XM Wi-Fi and incoming calls would sometimes connect but the phone would always revert to call failed disconnects on incoming calls. Shutting off Wi-Fi calling and then rebooting the phone would resolve the call disconnect problem. This was with carrier settings Xfinity Mobile 40.0.
After the last 3 months or so of just leaving Wi-Fi calling "off" because of the issues, I decided to see if they had miraculously fixed it.
Today (April 2) I checked to see if XFinity had updated the carrier settings. They are now at 41.0. I turned Wi-Fi calling on and the phone immediately locked on XM Wi-Fi and has not flaked out as before. Incoming calls have not been disconnected.
I learned previously that just turning off Wi-Fi calling without rebooting the phone didn't fix the issues. My iPhone XR is on iOS 13.4 with Carrier settings 41.0 (Settings -> General -> About -> Carrier). This looks very promising - I don't know if Apple fixed this in iOS 13.4 or Xfinity fixed their carrier settings. If the phone starts disconnecting calls again I'll update my post - FYI.
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