MTN upgraded my contract without my authority.

I had a Contract with MTN from 2018, which ended at the end in 2020. I received a call from MTN sometime in February 2021, the gentleman from MTN offered me an upgrade, I declined because I want to reduce my monthly expenses, as well as I just did not need the upgrade. To my surprise I was debited for the upgrade that I requested cancelled. I then reversed the debit order because I did not authorize the upgrade. About 5 months later I receive an sms from MBD collecting on behalf of MTN on an upgrade I did not authorize. At no point did MTN contact me to notify me of this ongoing and accumulated balance of the **********ly upgraded account. I called the call centre around July requesting for this **********ly upgraded account to be investigated, I requested for the call recording to be pulled and resolve the query for me. No one till today called me back from MTN I then sent an email to their complaints department over a month ago requesting assistance, I only received feedback 3 weeks later to say I don't owe money to MTN and they will resolve this issue for me this was a week ago. In the meantime I'm getting calls, emails and sms's from MBD collecting money on behalf of MTN. I complaint on their social media, I'm still waiting for assistance. I'm told to wait for another 7 days. NO ONE CARE AT MTN, my credit record is impated by this mess.