So I assume you have tried this: it seems to have worked for many.
The problem is caused by the nature of Virgin SIM cards which are Belgian in origin to allow Virgin to change network providers and give them access to roaming. iPhones don't seem to handle that well.
This problem has been round since the cards were introduced in 2016
If this does not work then I'm afraid you are out of luck
Settings -> mobile data -> SIM Applications (near bottom) -> tap "Roaming" -> tap "Select Mode"
I found "automatic" was selected by default.
Tap manual -> then choose National.
Now make sure your numbers are set to the correct format. If not change region to anything other than UK (Settings -> General -> Language and Region -> Region) and then change back to UK region
I'm a Very Insightful Person, I'm here to share knowledge, I don't work for Virgin Media. Learn more
Have I helped? Click Mark as Helpful Answer or use Kudos to say thanks