Claim for overpaid account

I have paid over R3000 for an account of R999.00 over 7 months, my mistake. I am owed R21 000.00. I have been with Vox for 6 years. Customer service has never been good but what is happening now is abuse. I am a consultant, I bill time. I can’t sit on the phone for more than an hour to get to speak to somebody and still not get help. It can’t be that the managers are so busy they cannot take calls. Waited 25min to get to talk to somebody who transferred me to management. My call having been dropped after 30 minutes from being transferred to management shows the same disdain by managers to customers that is shown by accounts call centre people. The amount of time they take to answer calls and the disinterested attitude on answering.