Cant buy a house because of R 580 that I did settle on a Vodacom contract (with proof of payment).

I have settled a celphone contract with Vodacom over a year ago and have the proof of payment on my bank statements. Yet this still shows as an arrears account on my credit score. I have phoned their legal department 5 times. They say that on their records it shows that I did settle the account and theat they would email me a letter of confirmation stating that I settled the account. To this day I have never received that letter. I cant buy a house because of R580 rand that I did pay to vodacom. How do I settle this?