R1750 deducted from my account with no explanation

I have a contract with MTN which has a debit order of R400 each month. In the month of January I had a debit order for R1750 that is unaccounted for. I am getting no help from the call centre as they are unable to give me a detailed breakdown of how the costs were incurred and i have still not heard back from them. I have also apparently started being charged over and above my contract amount for February for a sim card that i have not even used.