Vodacom
Final Demand, but accounts are paid in full

I have had a Vodacom account, which is paid in full. I have then converted the numbers to prepaid. All payments was via debit order. My last statement indicated a value of 0.00. I then got a final demand warning yesterday which says that I owe you an amount of 2328, 72 - which is incorrect. I then phoned the contracts department, which said I should speak to accounts, which then said I should speak to cancellations, and nobody could assist me as my accounts are on prepaid. I'm phoning all the different departments and nobody can assist me.