handing over an account i wasn't even informed abt

I had a contracts with cell c in 2011 when it ended they stopped billing me (debit order), I stopped hearing from them then, then today I receive call that I need to settle an account of almost R4000 in the next 4days, I don't understand how cell c failed to contact me as they have always did, my alternative number, email and physical addresses are still the same. why was I not contacted if my account went to arrears cause when debit orders stopped and they didn't say anything I was relieved that i'm done with them, but for them to only contact me 5 years later. I need some serious clarification