MTN Ignorance to queries

MTN South Africa has the worst customer care . I have been their royal client for 10 plus years as a top up contract client. Out of the blue they gave me a bill of R9 900. I have been asking them who opened up the line to have such a bill . Duration when the bill was acquired l was not even using the line. Tried calling 135 and emailing but no response from MTN instead they have resorted to handing me over to NUdebt and my credit record is negatively affected. On the email from Nudebt they wrote that they contacted me and they never did that and they also claim that l acknowledged the bill but refused to pay , which is not a true reflection of events. I want my contract permanently terminated after resolving the issue and l will never recommend MTN to anyone.