Complaint about MTN

My complaint is about MTN, my mother had a cel phone contract with MTN, she passed away in July 2021. I contacted MTN on 135 to cancel her contract in August 2021, after holding on for 40 minutes I eventually spoke to someone who said I need to send through her death certificate, which I duely did. Now an SMS came through saying her account is in arrears due to none payment. So another 30 minutes on the phone, only to be told to speak to the legal department, they transferred to call and another 15 minuets later they answer, I tell them the story and the line goes dead so after spending in total 2 hours on the phone the matter is still not resolved. I am trying to do the right thing, but it seems as though they don't care. How do you run a business like that, it's unbelievable.