Unkown oustanding debt

I wanted to apply for an ADSL line for home, I was told I owe them R752. which they are unable to explain as this amount is more that 15years old. I am not able to pay the amount as I don't know what it was for and I had prepaid line until last year. The call center referred me to the service center (Springs Avenues) which was also unable to assist as they said the system they used for the account is no longer valid. They are not able to explain the account and the service that is owed. if you call the number 10210 you have to wait at least 30 minutes for them to answer.