They don't cancel lines when they are supposed to

My parents have moved from Warmbaths to Port Shepstone and from PS to Durban (Forms were filled in to cancel the line and I phoned them to find out about any outstanding balances). In both instances the Telkom lines were cancelled but Telkom didn't do their job and now they are harassing my parents about outstanding bills. My father is 84 years old. He doesn't want threatening messages stating that his name is being blacklisted. Telkom must go check their recorded phone calls and credit the account. How can they in any case not see that another account was opened in a different place under the same ID? Stop harassing my parents!! Sort out your own mess.