Thank you. The O2 site is handsome but not the easiest to understand.
I followed your link and just for interest checked whether I could upload Wifi Calling on my iPhone 6s under the latest OS11.2.6 . Coincidentally, I just received a newsletter from the " IanswerGuy " who offers advice on Apple kit. They both refer to a feature button called WiFi Calling that needs to be enabled. (Settings/Phone/Wifi-enabled)
That feature button is not listed on my phone. Does that mean the service provider has to download firmware to enable it to work, or is there a problem with my phone?