termination of contract

I payed off my 2 contracts, terminating it before time last year and on the other account they told me the amount of money left was R1800 and I went to FNB and paid it off and now I keep getting letters saying I owe them R1800 and I am tired now, I am even in arrears because of it, I've called their customer service sooo many times and they keep giving me different numbers to call, if I call the other they give me other and also the other way around, this is reflecting badly on the bereau