No contract but debit orders

I have contract with MTN of data. The router started malfunctioning I removed Sim and inserted to the phone so I can still use data. I then used that Sim for phone calls. Went on to do advance then all of sudden they gave me open line contract they started deducting/debit order. I reported under Ref ***. I was told it will be rectified now I received invoice of R844.18