Thank you for your reply.
I had already looked at the web page you suggested and what prompted me to ask the question was: "Your SIM should already be set up to allow Wi-Fi Calling" and so I wondered if my SIM was not set up as I've had it for a few years.
However, I eventually got it to work by the old and tried method of turning off the phone and then switching it back on again a few minutes later. WiFi Calling appeared in the top left hand corner and I made a call. The second time I tried this the wifi connected but "WiFi Calling" did not appear. I've got a reasonable signal at home so this may have prevented it happening. I'll try it out when I have the opportuniy to access a wifi signal in an area of poor coverage and see what happens.
