NUDEBT Management Harassing Telkom client

NuDebt acting on behalf of Telkom Accounts is harassing people. My mom had a outstanding Telkom Account after she cancelled her contract with them in January 2019, They billed her for two months after cancellation. Nudebt send a SMS last week notify that she has been handed over for debt collection. I paid the amount on Monday 26/8/19, send a proof of payment to Nudebt Management. Since Monday we received numerous calls from them keep on bullying her to pay, and when she and I told them that it had been paid, they just keep on harassing her, telling her that they will sue her. I have contacted the Nudebt Management via Whatsup again send the proof, they notified me that they did received it, and they will make a note on the account. Today still they contacted my mother again keep on harassing her. This is not acceptable.