Not my calls

i had taken out and MTN contact in June last year for a TV and cell phone for R250. They gave me a sim card which i cut and threw away on the day. Today i found out a debit order for R650, R1123 went off. Being utterly irate i called MTN cc now and got through to 5 agents which i got disconnected and no return call, Rushed to the MTN store in Cresta who couldn't help but could suspend the number, which while talking to he best buddy from LG sitting next to her, she suspended the wrong number.Called the CC now and spoke to Elroy, who corrected everything but says MTN will not investigate as the number and sim belonged to me and i should go to the police or do my own investigations. I requested to know how much has been used so far on that number that i will be billed for end of the month and it totalled to R1700. I feel like crying..how can MTn not investigate who stole this number and i am getting billed for it...why?How do you treat your customers...