Telkom continues charging on cancelled account and wants R3,500

We cancelled an account with Telkom in Jan 2018. We have been hounded by debt collectors in the past 2 months as apparently we have been handed over for non-payment as Telkom continued to bill us for 18 months on this account and they want us to pay about R3,500. I have been in to Telkom and they cannot find the account on the system to log a dispute - seems it disappears when they hand it over. We had no communication from them and had no idea this account was still active on the system. The debt collectors are rude and told me to just pay and get over it! I am horrified. The lady at Telkom store told me she doesn't know what to do as the account is not coming up and that her head is spinning as she doesn't know what to do. I have all the proof of the cancellation documentation and final payment that was done in Jan 2018 but cannot find anyone who can help us sort this out. What a nightmare. Can someone from Telkom please help!