CELL C TRYING TO RIP ME OFF FOR A PAID UP CONTRACT!!!

I am not happy with CELL C at all .My contract ended in this year March or April,I confirmed with THREE agents on two occasions if there are any outstanding balance on my account and they confirmed no the line is on prepaid paid up and mind you i even went to two cell c branches and they showed me on the system the same thing R0.00 But now i get calls from cell c telling me now all of the sudden i owe +- R300 of which i don't know where does that come from.Are you trying to rip me off or what?.These are the agnets i spoke to Nokwazi from customer care,Shaun and Nakallo from retensions.