R99 sim contract

In December I took a sim card contract for R99 per month. A debit order was supposed to be deducted from my bank account.Nothing happend so I phoned and they gave me the telkom account nr to pay the money in. In march they deducted a huge amout from my account. I reversed the amount as I already paid the money. I phoned them but they dont know what is going on. I paid the R99 every month now they say I ow them over R900. I will go to telkom tomorrow to try and sort is out. Very useless service.