I think you gave yourself a good answer already that the disconnects could’ve been up to the signal. Another issue is if you have heavy network activity on your home network while you take a call. There’s a whole science to it (“buffer bloat”). With a few more days and many more calls from the same physical location you might get an intuition when Wifi Calls break up.
VoWifi is implemented by the system, not an App. The Dialer merely uses the system facility to use Wifi Call. It’s basically VoLTE after a VPN tunnel is setup to the carrier. All the “VoWifi Apps” I took a look at were thin wrappers to settings relating to VoWifi that can be accessed also without them. Any App you’d install could be at best again a Dialer that uses the system facilities.