Copy-pasting my previous reply from another thread in case someone runs into this issue.
I bought my phone from an Apple store in Japan. So they are definitely just messing around with you. Any unlocked iPhone should work.
Personally I would try calling again, and again, until you get to talk to someone that knows what they are doing. They literally just need to activate WiFi calling for your particular SIM, on their end
After much backing and forthing I asked her to "disable and re-enable" it on my account, and lo and behold I received the SMS confirmation activation (which I never received previously) and WiFi-Calling is now active and working!!
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So, Yesterday I decided to try and phone VM to sort my Wi-fi calling. I was on hold for around 75 Minutes and eventually got through to someone with good English. He did know what wi-fi calling is and advised that it was setup on my SIM, but then said that my phone isn't supported, it's a Samsung S10+. I told him that the VM help pages do state that it is supported and after a back and forward for a few minutes, he checked the Internet page and found it is supported.
I advised what has been said on here about deactivating and re-provisioning and he said that he was contacting 2nd level support to get them to "send a kick". It's now been almost 24 Hours and I still haven't got the new text and it still isn't working. He did say he'd activate the other SIM on my account, that's in a Samsung S9 with latest UK BTU firmware, it did receive a text but it's not working either
As we've already discussed just because it's an S10+ doesn't mean it'll work. As the help website does say, only the models where they've been supplied directly by Virgin Mobile are supported.
All of these devices support WiFi Calling if they are bought directly from us. If you are using a device that was bought from another network, or SIM free, we cannot guarantee that it will support our WiFi Calling service. If your supported device doesn’t have the WiFi Calling option available, make sure you have updated the software to the latest version and reboot your device.
In practice for Samsung devices that means the 'BTU' UK variant with Android 10 and the April 2020 security patch or later. Any older Samsung devices or other UK network variants may not work, and as your S10+ comes from China / Hong Kong it will definitely be configured differently.
Why your S9 is not working I don't know, but that would have to be investigated by Virgin Mobile.
********************************** I work for Virgin Media - but all opinions posted here are my own