I think your only option would be to make an official complaint.
This is normally 5-7 working days before they respond.
Complaint Review Service
Read the document here.
and write with your complaint to :
• Your name and address
• Your mobile and account numbers
• A daytime phone number
• A suggestion of how they can put things right
O2 Complaints Review Service
PO BOX 694
More information here : How do I make a complaint?
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