Closed account handed over to VVM

After being an MTN client for many years, I decided to use FNB's cell service when I moved my accounts. My MTN contract expired, that was December 2016, so it would easy. I closed my account at Cavendish Glen shopping centre in Kempton Park by paying the outstanding amount. I've never heard anything from MTN eversince.

All of a sudden I was contacted by VVM debt collectors to inform me of an outstanding amount. I went back to the branch where I cancelled my contract, and when they went on the system, they could find no evidence of any outstanding amount and told me that it might be a ****!

Yet VVM pestered me, mostly at the most inconvenient times, to pay up or be blacklisted. I am so frustrated as I have emailed MTN to ask for an invoice or statement for the outstanding amount...no response. I 've asked VVM for the same, but have received nothing up to date, only threats! Even guys at the Cavendish Glen MTN-branch have emailed head office and up to date we have received nothing. What am I to do????