Handed over to debt collector for a ZERO balance.

Dear Vodacom. I recently cancelled a sim only contract with you and paid the prescribed cancellation fee. You proceeded to debit my account with the correct final subscription amount, and I was under the impression all was good, as my final statement showed a zero balance and I have not received any further communication from you. You also cancelled the debit order you held. I am now being contacted by HP Attorneys, saying that you have handed me over for arrears.

What is going on? Please fix this.