Cancelled contract and no outstanding balance Vodacom wants me to pay more

I bought an Iphone at vodacom it was a 2 year contract, i had 3 months left before the contract ends, so i decided to pay the outstanding balance so that i can cancel the contract, and i managed to do that successfully They cancelled the contract and changed the vodacom sim card to prepaid. After 3 months i got a call from Vodacom asking when i am going to pay the R800. I was very angry because i asked the lady before i cancelled the contract how much money should i pay to cancel the contract she gave me the figure and i paid and the cancelled the contract. where is this R800 coming from. every month i get a call from Vodacom its annoying me