Account in arrears despite being on prepaid

<p>In 2016, I opted not to renew my Vodacom contract and switched to a prepaid package. The switch was confirmed by Vodacom. They debited my account for the last time and thereafter I bought airtime.</p> <p> </p> <p>I logged in to the online portal and saw a notification saying that my account was in arrears. </p> <p> </p> <p>How is it possible to be in arrears on prepaid on a number that I don't use? </p>