Unhappy client

My mom finished paying cell c in 2022 July but she still get phone calls from cell c saying she owe them. She was paying the account via debit and she even got the last statement that says closing balance R0. One of the consultant gave us email to send the statement but I got a reply saying that the account number they collecting for is not the one they collecting for. I asked them to delete my mom's number and fix from their side but even today she got 3 phone calls from cell c saying the same thing that she is owing. Can anyone advise what we can do for them to stop calling my mom cause all the time when my mom get this phone calls, she does not eat and gets emotional.