Non Existent Customer Service

I receive monthly a SMS's stating that my account is in the negative of R2,50, I try every month to get hold of customer services to enquire as to why there is this amount outstanding. My vehicle payment is on debit order, which goes off every month. The number they have ***is never answered, just cuts you off when you dial it. There was once I got through about a year ago, where I was told it was a clerical error, I must ignore it, yet I still get the SMS every month.