Thanks for the response, @Paulina_Z
Have you been able to put the SIM card into another phone to see if the same issue continues?
I confess I've not tried that one - I can do so however will need to wait until she returns from School this afternoon.
Can you tell us if this SIM is a part of Virgin Mobile, or if you were sent this SIM directly from O2 as part of a Volt package?
I believe its part of Virgin Mobile. I'm looking at my Virgin Media package, and it says "Volt benefits" are not active. Again, I'd have to check when she's back from school, but I recall the SIM itself being Virgin branded.
Let me know if there's anything else I can do in the meantime, oherwise I'll post an update when she's back from school.