On Wednesday, 31 July 2019 06:59:03 UTC+1, The Natural Philosopher wrote:
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.

My Pixel 3a states "WiFi preferred" under the setting for switching
WiFi calling on or off. It does seem to select WiFi whenever it
reasonably can be expected to. I use EE SIM-only.

John