My s8+ is open network from car phone warehouse and WiFi calling is fine once I turned it on at o2 it took 48 hours then WiFi calling appeared on the settings under call.
See the phone with WiFi on it in top bar
This is the chat with O2 (one of, they've been all the same):
Can you please dial *#1234# and look for the CSC entry?
Aishwarya • 6:04 PM
Zoran • 6:05 PM
Thank you, Zoran.
Aishwarya• 6:07 PM
Zoran • 6:07 PM
Aishwarya• 6:08 PM
Please give me a few minutes while I check this for you.
Aishwarya• 6:08 PM
Zoran • 6:09 PM
Aishwarya• 6:10 PM
Thanks for waiting, Zoran.
Aishwarya • 6:11 PM
Zoran • 6:11 PM
Unfortunately, your device won't support Wi-Fi calling as your device was purchased by third party and the CSC doesn't include ‘TEF’, ‘O2U’ or ‘OTF’ which is required for Wi-Fi calling.
Aishwarya • 6:15 PM
I wish there was a way to send you this firmware patch. However, it is installed by Samsung itself when they provide the device to O2 for selling.
Aishwarya • 6:16 PM
Login to mymobile.o2.co.uk then go down to near end to WiFi and 4g calling click it and it lets you tun on or off WiFi and 4g calling on your account. Turn on takes about 48 hours but then WiFi calling appeared under settings in the call menu on my unlocked none branded s8+
Now this is something completely different. I don't know the answer, the quote about the CSC can also be found in O2 online help section, I believe that is where this customer support person from the chat transcript I've wiredq, actually have the information from. And now I feel I should escalate this further, but not sure how...
However, thank you very very much for giving your case to help others, I just hope I'd be able to use it in order to, at least, get some other explanation...
I was on the trial early release of orio and it was on there too the last update before last fixed WiFi calling from switching off after a reboot. After the last update I had to go switch it back on under settings on calls but it's fine since and works well as I am in a low signal area and WiFi calling let's me make and receive calls. Main thing is if it's not switched on at the provider then the option for WiFi calling dose not show on the menu under settings on calls my fires only showed up after 3 days of switching it on at the o2 website.