Debt collectors received an incorrect amount due

When my friends moved they did not receive an account. First thing is we get a sms from debt collectors. After months of e-mails I now realize that Telkom did not credit an amount previously paid. This amount was paid before they moved and the service was terminated. The debt collectors (separate complaint) seem to be unable to change the amount handed over. In the meantime interest and cost are piling up. I have time and again confirmed that I will pay the outstanding amount as long as it is the correct amount! I have spent hours to try and resolve this matter and are now fed up