To clear up any confusion, I did some reaarch
WiFi Calling will activate on phines that support it and mobile networks that support it when (a) there is an adequate wifi signal (b) the mobile signal strength drops below a given level.
My phone (galaxy A3) (and others to judge by the research) tends to hang on to the mobile signal rather than use the wifi when it can. When the signal drops the phone will open UDP connections to ports 500 and 4500 - these can be seen as NAT sessions on my router - and a 'wifi calling' icon should appear in the phones notfication area depending on the phone and the version of android etc.
So long as these NAT sessions are open the phone can send and receive calls and SMS messages over the internet.
The other thing I learnt from the research is the tech support at most mobile phone companies is worse thanh used toilet paper.
Nearly all wifi calling problems were solved on the fira by other users, not tech support personnel