It’s very easy to reproduce time and again. Initially I thought it was only calls to and from other Xfinity/Verizon subscribers but that’s not the case. I can reproduce when I go into Airplane mode on the phone. Interestingly enough, while in Airplane Mode WiFi calling works just fine. The issue manifests when I leave airplane mode. The carrier correctly changes over to Xfinity Mobile from XM WiFi but it would seem that something in that re-registration back into the cellular network doesn’t quite work. It appears to be in cellular mode but behaves as if it’s in Limbo where it’s able to notify the phone that there is an incoming call but once it’s answered it goes to call failed until I go back into the settings and toggle the WiFi as well as toggling Airplane mode. I did some other testing earlier on where it seemed that it could also be related to VoLTE. So I may try some additional tests leaving WiFi calling on but turning off VoLTE(not sure if that will work or not). I find it a bit strange that this issue has persisted for so long especially since I haven’t seen this or even heard of such a pervasive issue on any other carrier, especially Verizon which if it was a network issue I’d expect it to manifest there as well. Leads me to believe it has to do with the carrier settings in how Xfinity is coding their WiFi calling rule sets or something.
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