MTN Contract Headache

My contract expired at the end of September but MTN has continued billing me. I contacted the call center to query the debit order and the story was I moved to zero billing also there a sum of money outstanding. Who instructed them to change the option? How do you bill a customer when the agreement has reached an expiry date. Then they have the edacity to tell me my account has been handed over to collections.

Angry loyal customer for 21 years.