Telkom bill 1 year after contract cancelation.

Telkom has recently sent me an outstanding amount bill of R573, 1 year after I had closed my account with them. I had taken out a cellphone contract with them in 2018 which was supposed to end in 2020. But I decided to payout the account and cancel the contract. I was given the balance which I paid in August 2019. I have had no issues until recently when I received a bill of R573 and when I refused to pay it they billed me for refusal to pay.. What sort of business is this.