MTN data line

I have app**** for a DATA sim card with MTN,they issued me with two sim cards off which I called them regarding the two sim, when they investigate through the voice records, indeed I ordered one sim card,they then promise to cancel the other SIM card off which it took them months to cancel Few months later I called MTN again and they told me that they will credit me with the money for the SIM card which they were deducting through out the process which it did not happen,now I'm told I'm owing MTN nd my account is suspended nd I'm listed on credit bureau