OK, she didn't know her PIN so rang virgin who told her to dial *#67# and then ring the 15 digit number that was displayed.
The number displayed was her mobile number with +4405 in front of it - I know the +44 is the UK dialling code but what the heck is the 05 (then it had her full number incl. the leading 0) - Dialling this came up with an incorrect number...
I then asked what she did dial for voicemail and she said 222 - this came up with the same error as before so I tried just dialling her mobile number from her mobile and it accessed her voicemail without a problem!
I've now programmed that in as her voicemail number so she can just speeddial it and all is fine.
Thanks for your help
He who asks a question looks foolish for 5 minutes. He who doesn't ask a question remains foolish forever.