Failure to cancel contract

I have a complaint regarding MTN and their service. I closed an account a year ago through a phone call. I was told by the consultant that my account is closed and that I don’t owe anything. Now, I was called by the billing department that I have an outstanding balance. I gave them a reference number for my cancellation. They promised to escalate and investigate the matter. However, this was not done and I never received any communication. Since then, I have been getting calls from debt collectors that I am owing MTN. I really wanna matter resolved.