This is just since the March update, it worked perfectly well before.
Power on phone - both VoLTE and VoWIFI work.
After an hour or two WiFi calling doesn't work and phone reverts to 3G when making or receiving a call.
So after running for a bit VoLTE and VoWIFI stop working.
I know it's really fun and exciting that after every Nokia 5 update you are treated to a new failure of some sort or another - what will it be this time? - but it would be much better if you actually tested the changes that are made to make sure the dammed thing still works!
I decided to try some Voodo. So I turned WiFi calling off and the powered off the phone and left it for a while. Then turned it back on and then turned WiFi calling back on. It's working properly again now.
So it could be a problem that happens if the phone is powered on when WiFi calling is on. I've recently switched networks to get VoLTE / VoWiFi so this glitch may have been there before the March update as that was the first reboot since turning WiFi calling on for the first time.
In about a weeks time I'll need to reboot the phone because of the "Apps closing in the background" problem so I'll see what happens then.