Final Demand from Vodacom relating an overdue Staff Account

I hope that someone at Vodacom can eventually assist me and get the following issue sorted, as I have tried all avenues without success. I let Vodacom last year February 2018. I requested that my Staff Line and Private SIM be converted to Prepaid, this was done. A few months later I received a Final Demand from Vodacom relating an overdue Staff Account. How is this possible if all amounts were settled before my last salary payment and I also gave Vodacom instruction to debit any additional amounts from my personal bank account. I started to receive SMSe and calls from a debt collecting company MBD INC. It has now come to a point where they are invading my privacy, totally unacceptable! Not only that, I was also told by one of their call centre agents that I have been listed with ITC. I tried to reason and explain to their staff that something must have gone wrong, as it is impossible that I own Vodacom any money - however they kept going in circles and demanding payment, that I refuse to pay, until I have a proper explanation around how this amount came about. What is more confusing and concern is that the Vodacom Letter of Demand shows a different amount as the SMSs from MBD INC. I therefore would like to request a FULL AUDIT and RECON on my account from the day I left Vodacom last year February 2018. I contacted the Vodacom call centre in order to speak to someone in Collections. After about 5 attempts I eventually spoke to someone, however the person was unable to assist me. I was then asked to call the Vodacom staff help line, however no success, as my mobile number is not recognised as a staff line. My concern is that if something like this has happened to me, how many other customers or ex-employees might be affected.