MTN Contract cancelled - still being charge cancelation fee

Good day - I cancelled a contract with them on there terms 20 days before the contract end date as per mail from them. I have phoned several time with all refences numbers but they keep on transferring me and cutting me off, and closing the queries every time with out feedback to me to me... this is not fear - my account was never in arrears or late payment... I can not seam to resolve this issue on my own...please I need some help...they are not going to help me... on there side one department is blaming the other - retentions said it billing...billing transfer me back to retentions then they cut me off... phone over and over they just don't answer.