After Ive cancelled the contract I am now owing

Last year October 2014 I cancelled my contract with cell C it took me several phone calls speaking to different people and different department until finaly somebody assisted me and my contract was cancelled. Early December 2014 I was told if I dont pay the balance they are going to suspend the line after I paid the balance which was R50.00 (Ref *** ) and email copy to the email addres provided I had to later make a call so that I can be back on line they then put my line back. Now yesterday(5 May 2015) I received a call from cell C regarding my outstanding balance of R271 when i asked the lady what is it for she said its the balance that I am owing when I explain to her she said she can see from the \SYSTEM"that R50.00 was paid but doesnt say for what I find it very interesting with this people who always telling us about the system so now for almost 6 months Ive been on prepaid all of a sudden I am owing if the Ref number records details on\"" SYSTEM\"" then i dont think Ill be called and told I have outstanding balance"""