I just recently noticed this issue as well. Noticed it with FaceTime call and with Whatsapp call also. WeChat calls were on and off as well. This is with cell data turned off and me sitting right next to wifi router as well. Wifi router is literally sitting on my desk so reception is perfect. Have Gigabyte download speeds as well which I get half of
I’m having the issue on iPad of it forcing a FaceTime audio call instead of a real call. Most people aren’t used to answering FaceTime alerts, so it’s a big issue.
I'm not sure if this is related but I noticed going about my usual routine my iPhone's LTE & WiFi felt a lot more patchy compared to its normal self. That includes WiFi calling seeming less stable than on iOS 12.
Not extremely bad, but I was certain it was different.
In my experience, it is been a hit and miss since its introduction and I never had it working well under iOS 12. IMO, there are just too many factors involved, like different routers, connection speed, line quality, etc. Unreliable stuff.