Telkom and Debt-Inn Harrassing ex clients for payments already made

My grandmother had a contract with Telkom. In 2020 she decided to cancel her contract in September. On cancellation she was informed she would need to pay for one month extra and then the line will be cut. On the 31st of October the line no longer worked. Telkom proceeded to charge her for November and December. At the beginning of 2021 Telkom handed her over to Debt-in who co tinue to harrass both my grandmother and grandfather calling 3-4 times a day requesting unpaid amounts from Telkom ranging from 2019. Upon showing proof of payments for those amounts and days Debt in comes in with a new unpaid amount for another month. Which is impossible. Debt-in continues to harrass. I've had to now pay a total of R344.77. This is ridiculous.