MTN can’t tell me who has my account and who authorised 3rd parties to make debits from account

MTN has handed over my account to the debt collectors and can’t tell me who they handed it to. According to their records my account is closed and after phoning the debt collectors, they also have my account listed as closed. MTN also gave me a different debt collectors details and after they checked, my account is not with them too. I have sent emails, called and put through to different departments but still no resolution for the past two months This caused a blockage in me making certain purchases and affected my credit record.