Billed for settled account

I took out a contract with cell c in 2014. I settled the account in March 2015 due to not receiving any good service since day one. Now, in July 2015 I received numerous messages stating I owe cell c R545.00 for February statements. I requested a full settlement letter, which came to just over R3000, amount was paid in full. I spoke to cell c manager at galleria and he treated me like I was making this up, \you clearly don't understand what I'm saying to you"";but if the number is on prepaid. How can I owe money? I also haven't received a statements since January 2015. This place is a joke. """