it sounds like International call barring may be on in your account.
Log in here: Your Account > click on the "Plan and device" tab.
Go to "Manage my services" > check that "Roaming" is set to ON & "International call barring" needs to be set to OFF
If you have to change these settings. Allow time for your account to update then reboot your phone by turning it off & on again.
Hopefully this may be your problem and you'll soon have it sorted.
If not, you could try here for help: Help and Support - Contact Us > Mobile > Mobile not working > Chat online.
The advisors are often busy though so you may not get connected straight away.
Alternatively, if you can borrow a phone to call Virgin Mobile. Their UK support line number is: +447953 967 967
If you want to say thanks > click 'Kudos'.Answered your question? > click 'Mark as Helpful Answer' also.