We're experiencing exactly the same problem and also upgraded the phone, from an S6 to an S20 thinking it was the handset that was the problem. Now we realise it is the network, we also had a new sim with the new phone.
It doesn't seem to be related to WiFi calling as the S6 didn't have it turned on, but it is turned on, on the S20 but we experienced the same problem with both phones. This has been an expensive way to attempt to fix this problem, we wouldn't have upgraded, if we'd known this was a network issue and not the handset.
We also lost mobile phone service for over 24 hours during the switch between phones, Virgin Media claimed this was due to an "outage in your area" - "We apologise but there is currently an outage in your area. We are working to get this fixed as soon as possible." Which I find hard to believe as we were told there was an outage in two locations either side of Cambridge.