Accounts incompetence

I had a Telkom lite data account, last year the contract period ended and because I decided to go with fibre, I did not continue with a new lite contract. I went into my closest Telkom store to cancel the account, the lady assured me that because it's the end of the contract period and I do not want to extend it, it will be cancelled automatically and the last debit order will also stop automatically. All this happened exactly as she told me. In July I received a letter of demand from Telkom for the amount of R7649. 66. For this account. I've been phoning, trying to get them to sort it out, writing letters, to no avail. Now Telkom handed it over to some immature debt collector company that phones me 5 times a day and when I answer the person says, pay me the money, when are you going to pay. They are really rude. This contract ended, I even went into the store to make sure it was cancelled, it ran on a debit order that was cancelled by Telkom at the end of the contract, I was without internet for 2 months waiting for the fibre to be installed. I am a current customer of Telkom. Please sort this out for me Telkom.