nill owed but handed over .....

i wanted to go renew my contract with cell c and was told that i have an amount outstanding so i could not . i then contacted cell c that said i dont owe anything . a week later i got an sms from cell c debt collections saying im being listed on the credit bureau . i phoned cell c again and was told i dont owe anything . i asked them for a letter to send to the nudebt collections . The agent said i must talk to the legal dept ..... i got through to them now one month later after being cut off treated like ******* by very unprofessional staff .apperantly i owe them R528.38 from my previous contract that i cancelled in Jan 2014 cause they could not deligate the proper payments between my 2 contracts . i remember being told that time that my contract would not be closed untill all payments on this contract was up to date which i then did . Two years later i now suddenly owe them money again . when i say to the agent i dont owe the money , he says my system says you do so you must pay it ...PLEASE SOMEONE HELP . PS why was my contract closed and cancelled if i still owed money 2 years ago .