This is just an update and summary.
Called Sky mobile when my new phone did not connect for wifi calling (was ok via mobile network when away from home). Had 3 conversations over 2 days. Was told nothing for me to do, once I allowed wifi calling on my settings (but no such setting was available on my phone). More googling, joined samsung community, explored all the "red herring" options to set wifi calling. Went back to Sky, told me to get in touch with Samsung as the option was nowhere to be found, posted on a couple more buletin boards (no replies), went to sky 3rd time & told to visit Cardiff support centre. (My son went a couple of days later for me as he was there on business). They were of no practical help and suggested going back to the retailer in case I'd been sold a "foreign" S7 phablet else it must be a Sky problem, but could give no explanation or suggestion about why the "Wifi Calling" option was nowhere to be found. Did a factory reset, in desperation - blew away all the diagnostic apps I'd downloaded, and loads of other stuff (just realised why my wake-up alarm failed for last 2 days) - but still no joy.
Called the retailer, he said no-one had complained about any faults on his latest batch from Samsung but suggested I try it on another network (not many S7 users need wifi calling, it seems). I did the opposite - I gave my son my sim card, and although he rarely uses wifi calling on his own Pixel, with my card fitted, wifi calling did not work.
Got back to Sky support, now armed with the fact that my card did not work in a proven phablet, got put on hold for 10 minutes while two (or more) techies had a chat, and WOW! my phone rang. It was the hospital, wanting to change a appointment. Eventually, Sky came back (still on the landline) and said they think it might be the sim card at fault, so had ordered me another. I told the guy that I'd just had a call on my phone, but he was not really interested as "new sim was the answer". I tried a few calls that evening to no avail, and began to doubt I'd had a conversation with "outpatients dept". Following morning I called the landline with the S7 - it rang! Did it again - it rang! Called my son - wifi calling was working. Took grandkids to school, came home to find new sim card in letter box. Called son again to let him know. called others. New sim card still in the envelope. My settings now shows under "Access Point Names" that "Internet" is "mobile sky" - but as far as I can tell that is the only change. There is still no setting option for Wifi Calling on general settings or on Phone app setting - so if like me you have a new S7 Edge running Oreo (Android 😎 and maybe earlier versions, do not worry about the option not being on your phone - it is a Red Herring (does not exist).
For me, all I can say is that having been bounced from Sky to Samsung, Retailer, and Sky, that it was my last encounter with Sky Tech Support thatsolved the problem, even though the Sky Guys had no idea they had done so - they clearly pushed a number of buttons and one of them worked - but without persistence, I would now be shouting at the supplier, who clearly was not at fault, nor had Samsung dispatched a faulty device. Now I have to decide whether to switch to the new sim card and risk upsetting my phone, or stick with the old one that now works.
