Debit after cancellation

I had a cellphone contract with telkom for a duration of 12 months ending on 30/10/2020

I requested cancellation of the contract on the 18 October 2020 on the service portal and I received an email and an sms to confirm that the request has been received and someone will call me back. Someone called and I advised that I dont need the contract anymore and it has to be cancelled, the explained all the cancellation terms (30 days) and all was fine then then I received an sms on the 28 October confirm that my account was submitted for cancellation. Last debit was in November which was a double debit I guess for October 2020 and the 30 days cancellation notice. I checked my account in November 2020 and I was no longer getting data airtime or minutes and I assumed that the account has been canceled as per the SMS's. December 2020 there was no debit then in Jan 2021 they start debiting my account again. I reversed the debit as I assumed it was a **** debit. This morning I get a call from debt collectors saying I owe telkom, how?