I have the same issue. My wife and I both have iPhone 8 Plus’s and ever since switching to Xfinity mobile we have had nothing but problems trying to FaceTime. Every time we attempt it, it gets interrupted with “poor connection” and ultimately freezes. Tried both on my home WiFi network (speedtested at 200mbps) and on LTE. All settings are up to date, thought maybe it was a hardware issue. Any idea how to fix this?