Are you sure that you have active roaming?
If you log in to your account > click on the "Plan and device" tab from the row near the top of the page.
Go to "Manage my services" check that "Roaming" is ON & "International call barring" is OFF
If this was your problem and you have to change the setting. Allow time for your account to update then reboot your phone by turning it off & on again.
If you are still unable to use your phone you could use the web chat option to speak to an advisor.
Or consider borrowing or using a pay phone to give Virgin Mobile UK support line a call on: +447953 967 967 and see if they are able to help.
If you want to say thanks > click 'Kudos'.Answered your question? > click 'Mark as Helpful Answer' also.