MTN Blacklisted wrongly

So today, I am driving next thing my phone rang. A women says hello we are debt collectors from MTN. I was so shocked since I didn’t know I owe MTN. In Mid January I called MTN asked them to give me quote on early cancellation. They gave a figure, I went to their outlets branches paid everything in full and called in for a formalized cancellation and Immediately migrated to prepaid the following day.

Next MTN thing on the 25 I am getting a debt order from MTN for what I don’t know. Called in and I was told stories on how someone didn’t do their Job and of which it wasnt my problem. I did what I was told and what was expected of me. So today I found out that mtn blacklisted me after i had paid up and cancelled my contract. You are unable to get hold of their legal department who houses *********** people working there. Nice one MTN!!! I am so sad and angry with such treatment. Ive been a loyal customer since the day I joint and never missed a payment on any of my contracts with you. What I’ve realized is MTN are bullies, especially on poor SAns. I need help to lodge a dispute at the consumer protection dept as this will ruined my good credit rating because of Mtn's *********** staff!!