Occasionally it won't connect to my home WiFi - just remains in LTE status. Not really sure, but I'm guessing this might be a VZW issue and not necessarily an issue with iPhone or iOS. I used to have AT&T and it always connected (same exact iPhone 7+ unlocked device, just swapped SIM). I never had this issue until i switched over to VZW.
Edit: Wait, I just thought of something...maybe if the VZW signal is really strong (which it is in my area) the device will prefer that connection instead of the WiFi calling. Maybe mine always connected to AT&T WiFi calling because their signal is not very good in my area, so WiFi calling was the better option.
I don't know if this is the case, maybe someone else can comment if they have more knowledge of the WiFi calling feature.
I have been having problems with both wifi calling and FaceTime audio over wifi. With a regular phone call, I can hear the other person just fine, but they tell me I'm breaking up and can't hear me. I toggle wifi off for it to go back to LTE, and that fixes it.
With FaceTime audio, both ends of the call are unreliable. It will cut in and out. The same fix works: turn wifi off and use LTE.
I could understand this if the wifi connection was slow or unreliable, but the latency is much lower and the speeds are more than sufficient.