mtn debit order surprise

I took a contract with mtn in 2019, i asked them to curb the payment to an amount of R200 every month, this month Jan 2020 the debit order went off at the amount of R765, I called them and asked what was going on and they told me i had an open contract of which I responded I don't have that and i know my sim card was lost but I don't understand how they allowed the calls pass the curbed amount of R200, i was given a reference no and today the 4th Jan when i called to find out the response was told the case has been closed with no explanation, kindly assist in getting to the bottom of this and mtn to refund me