MTN bills me for 8 months and hand me over after I cancelled contract and return faulty handsets

I agreed based on telephonic agreement to purchase a contract with hand set from MTN. Once the item arrived, the device was found to be faulty and due to this I return the handset to a MTN dealer in Groblersdal and requested the contract me cancelled. I called several times explaining this and ensuring the hand set was returned to MTN. I had confirmation the handset was returned, but then they refuse to cancel the contract or the monthly subscription on contract and the handset in Oct 2018. For 8 months they continued to invoice me and show it as outstanding to them. The non payment was also recorded on my credit records as NON payment - although I did EVERYTHING i should, but MTN continued to bill me and handed me over for non-payment. As mentioned this effect my credit record negatively and impacted other applications since then. MTN made no attempt to contact me or resolve the matter although I contact them several times and tried to make this clear to them.