Thank you for the clarification re the way O2 handles the call. It's almost like the eSIM number is a number translation service number with your 'substantive' (phone) number the target. I spent 2hrs in O2 shop. Assured me from a network level all was good, settings all appeared good but reading your message, the test was the issue. O2 store put the phone into flight mode and watch duly said 02-UK with signal indicator but emergency calls only when we tried to make a call. This led us to think there was still a problem. I guess there was - the phone wasn't connected to the network. So to test it, I move phone and watch far enough part to not piggy back (phone connected to network). Then try. Hopefully that will save me sending the watch back. It isn't massively intuitive from O2 or Samsung but hopefully I'll get there thanks to this community👍. Thank you to OP (I hope you are now sorted) and Papashaun for the explanation.
papashaun, looks like I'm going back to Samsung. I have a work mobile so I went out for a walk and left the phone linked to the eSIM at home. My watch displayed network provider, 4G and signal strength indicators so all seems good with network provider. I could send and receive SMS and WhatsApp messages. Phone calls not so much. I made a call from the work number, nothing would show on the phone until a missed call notification came up. I tried to return the call, get a pause and then the emergency calls only message popped up. Confused. It's half working. I exchanged the first watch under 30 days programme as a spring in the bezel popped out. Now it looks like I need to exchange this too. I'm stumped. Operator confirmed everything is fine their end and as I can send and receive messages over 4G independently that would appear to be correct. So frustrating, so near, yet so far.
papashaun, thanks for the response again. Will double check Google but most references I saw led me to this community. And I can see why. I think I've checked them all here but will check again and sending the watch back for a replacement is a pain and probably isn't anything wrong with the watch. But missing a tiny crucial step somewhere. eSIM is all set up good, yes calls and text on other devices selected. One last search whilst I'm on hold to return the watch. Thanks again, I really appreciate it.