Hello bumblebee83
Sorry to hear of the confusion around your mothers account and the O2 SIM, we understand the confusion and appreciate you taking the time to raise this via the forums.
It may be that the SIM was part of the Virgin Media deal taken, these are branded as Volt packages and include an O2 SIM card, this will all have been stated on the original contract and paperwork received regarding the package.
We don't have access to O2 systems so to speak to them you would need to contact them directly, their contact details can be found here.