Being billed on a canceled account

We received a notification today that the bill for the contract we with Vodacom was handed over. Which is strange, seeming as though it comes off via debit order. So I wait after requesting the contact call me in order to find out what was going on. I then am told that a contract which was canceled in or around December, was now over due in excess of over R7000. How is this now possible when we were told that our final installment would be taken off after our cancelation, which was taken off via debit order. Since the cancelation, there has been zero communication from vodacom of this alleged owing. Nothing has been sent. This is absolutely ridiculous, especially considering that the account has been canceled last year in 2019. Someone needs to contact me urgently to sort this out. I've never had service like this from vodacom, it is highly disappointing and disheartening, especially considering that we have been clients of them for numerous years.