Telkom/NU Debt unauthorized debit order

Not happy at all. I have been debited about R772... and I don't even know what is it for. I don't have any contract with Telkom or registered for any of their promotions/deals. I called Telkom and was advised to call NU Debt to find out about this debit order that went through my account. NU Debt contact number rings till number gives an engaged tone.