Payment harassment

My mom was an MTN customer. She passed away in Feb23. I started receiving notices of payments outstanding on her account. I called MTN and advised them of her passing. I settled the outstanding amount. Some 2 months later I continue to receive SMS' threatening an adverse listing if I don't pay. I went into an MTN branch which confirmed that she does not owe anything. Her contract has been cancelled. I have called the number on the SMS ***) a number of times and left messages. No-one calls back and I continue to get these distressing messages. Beyond unacceptable.