Outstanding money

I recently discovered that I am in arrears with MTN for R430.96 for a contract that expired more than 3 years ago. According to my knowledge, the account was up to date and I wasn't owing anything and I haven't been contacted since the contract was terminated. I have tried to contact their Corporate office, as I was advised by the call centre agent, but their lines are just ringing unanswered. I work in a very competitive environment and don't have time to sit around and hang on for no answer. I have a really bad history with MTN and they have not made any effort to show me that I am a valued client. This is really atrocious service from such a big company and I would strongly advise other clients to do business with them.