Telkom threatening me after a fully-paid contract was cancelled

I had a 2-year LTE account with Telkom which I cancelled early in 2021. The account was paid up by debit order every month. The contract had expired when I cancelled it. No further communication was received by me from Telkom, because, hallo!!!! The account had been cancelled. Suddenly, out of the blue, I am getting threatening SMSs, alleging the account is in arrears. It was never in arrears. Upon phoning Telkom, I was told that I owed some random amount of R200-odd (the actual monthly fee was around R900) but this amount had mysteriously escalated to R1110 - thereby breaking the In duplum legal principle. This is, again, shocking incompetence from Hellkom.