I have an Ecotalk sim, which uses the EE network in the UK. Because we have a very poor mobile signal here I’ve enabled WiFi calling, but it only works intermittently. I’ve switched to preferring WiFi over mobile to try to help, without success. WiFi calling comes on for maybe half an hour, then goes off. It sometimes switches itself back on a little while later. Disabling WiFi calling and re-enabling it works, for maybe another half an hour. Could this be an issue with the sim card, our WiFi, the phone (FP5) or the network?
One thing you could try is putting the phone into flight mode and then turn on Wi-Fi. Then at least the mobile connection cannot influence the phone’s Wi-Fi calling connection. If that’s stable then I’d assume the reason is more on the network than in the phone…