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


--
If you tell a lie big enough and keep repeating it, people will
eventually come to believe it. The lie can be maintained only for such
time as the State can shield the people from the political, economic
and/or military consequences of the lie. It thus becomes vitally
important for the State to use all of its powers to repress dissent, for
the truth is the mortal enemy of the lie, and thus by extension, the
truth is the greatest enemy of the State.

Joseph Goebbels