Poor Service

My contract ended in December 2020 and I informed MTN that I am not renewing. I got a confirmation SMS that I am now on prepaid. I was surprised to receive an SMS on 31 December 2021, a year later that they have not received payment and I must use the banking details on the invoice to make payment. I tried to call MTN and no one was able to help. I then sent an email to customer service on 3 Jan to find out what is this money for. I was referred to the retentions department. It's been 3 weeks now and response. I have sent a follow up email today.