Frastrated Customer :( :(

I'm not sure how I feel about MTN right now, I took out a contract and everything was fine, but I didn't want to renew it when my contract ended, so I called them to cancel, which they didn't, and when the end of contract was due in April, they still didn't cancel, and I called again in June, but they still didn't cancel, so I blocked them from deducting from my account. I received a final notice at the end of September and contacted again to inquire about and cancel, and to my surprise, I just received a phone call from debt collectors stating that I had been given over to them.This is quite frustrating because it has a significant impact on my credit score. But how much does this cost?