Handed over without prior notification

Contract taken out in nov/dec 2011. 2 years later i got a call to upgrade in oct 2013. Approximately 6 months ago i was called by debt collectors to say i owe Cell C money for non payment on my account. When i called cell c they could not pick it up. I brushed it off. A few months later i managed to speak to the legal dept with cell c and they said when the upgrade was instituted, 2 months were not paid for, however, i had never been contacted regarding this nor billed for it. This was no fault of mine & when explaining this to the legal dept he was rude & told me i must contact the debt collector's dept & pay. I initially needed to pay like R500 and something, now i need to pay legal fees of over R1200! I am now listed as a bad payer on credit bureau, had i known the amount was outstanding i would have paid it. Further more, why was my line not cut if i owed them money, cell c is quick to cut the line if you owe them money. i am disappointed as i was with cell c for 10 years and this year ended both my contracts with them due to bad service like this. I would like someone to look into this & assist me further. today i received another call from debt collectors demanding money