billing

mtn has debit R840.0 on my account without my authority, i had a contract that expired with them and i was told that i will have to pay R50.00 to keep my number, i have never defaulted on my payments, and i dont understand why they are debiting my account with such amount, because contract that i had with them was for R109 a month and it expired after two years, i want my money back, i have been calling them and no one is answering their phones