Terminate a expired contract due to death

On the 11 of August 2020 I email Cell C Kollonade to cancel my mother in law's contract because she passed away in March 2020. I attached the death certificate and all other documents requested by them. I have followed up several times with the manager of that branch. He only gave me a *** until now this contract is still debited against my deceased mother in law's account. I am very unhappy with them. My mother's account is running now in debt because of their poor service.