Poor customer service from MTN

I had an MTN contract years ago(2011) and did not use it anymore so I cancelled it but for some reason it was not cancelled. So in last year I recieved a call from Debt collectors saying I owe MTN money.I ask them how much and I paid them.I did not even get a settlement letter.It even reflects on my credit score.Even now after I paid them in November last year they say I still owe them money.they gave my account over to new debt collectors which never contacted me.I tried to contact them with no success,Went to a MTN store who also sent a mail to their legal department with the proof of payment and still got no feedback from them up to now.No phone call or e-mail.Don't know how to resolve this issue anymore.