Update: have learnt that wifi calling only works when I make the call, but not when people call me. So when people call and I am at home, they get message saying my phone is disconnected, or busy signal. What's weird is that this has only been reported by people trying to call me for the last few weeks - I've been in this house for 4 years.
So what's changed?

Surely if there is no reception the call should at least go to Voicemail? Telling people my number is disconnected is terrible.