Refuse to accept cancellation with Vodacom

I cancelled my contract over a year ago with Vodacom and handed me ove to HP Attorneys who is continuesly harassing me for payment. I do not even know how much I owe but I must pay... I sent the last invoice from vodacom to show I owe nothing, I sent the email I received from Vodacom to say my number is now prepaid... When I phoned the attorneys I was not helped, when I phoned Vodacom I was told to speak to Tobi the robot assistant... How do I get rid of this?