Deke hello let's see if I have this correct.
You stated that you attempted to make a call on the 4G network. Was this a data call WhatsApp, Signal, Apple or similar or a regular cell call?
If it was a regular cell call or WhatsApp call it should not use your data allowance.
No by default software will always run automatically and in the background and use you data, unless you manually edit their settings to disable or off. This will always use you data allowance up even when you think that you've only open say just an email app say twice for the month and you data allowance has ran out or run low.
Now that said if the call and data is some feature of iPhone I'm at a lost as I don't use them.
So let's have a rerun:
You have unlimited minutes to make calls plus 1 GB of data.
With unlimited minutes you should be able to call all the standard numbers land-line mobiles in UK, or another numbers that you don't have a bar on.
The other thing to check has you monthly packed be changed by Virgin without you knowing? ( If Virgin changes your package they'll always e-mail you well before the change over)
All done and said if after you've checked everything and it seems ok you'll need to take it up with CS ( Customer Services) good luck there.
Joe