Thanks for your help.
I should have said in my OP that I had thought it was maybe something to do with WiFi calling, but it couldn't be as WiFi calling was turned off. Also the graphic in the icon other than the number and handset was not exactly the WiFi symbol, and this is the other thing that made me suspect it was not to do with WiFi calling.
However, for some reason I hadn't considered that the '2' might relate to my second SIM card until you mentioned it, even though it should have been obvious!
Thanks to you, with my new found knowledge of the icon relating to SIM 2, it helped me figure out what the icon related to. It was VoLTE calling (Voice over LTE) rather than WiFi calling.
I didn't have a recollection of seeing the icon before but it may have showed up before and previously I just didn't care to figure out what it was for. The reason it showed up this time whereas it wasn't showing before, is I entered the country that SIM 2 served, then assigned some funds to it to get some internet data which allowed me the option of making VoLTE calls, which was turned on. Whether this is a new service that is now being provided or as I said before I just didn't care to take any notice of the icon, I don't know.
The options for VoLTE calls is found under 'mobile networks' which itself is found in the 'connections' section of the Settings menu.
