Unacceptable service on a simple extension to a rental agreement.

I hired a car at OR Tambo and wanted to extend the rental period by two days. I was informed that the rate I had was a special and now I need to pay almost double the normal rate. After holding on for more than 25 minutes I got cut off. I called the call centre 5 times and couldn't get through. This is appalling service. A simple request can not only be processed but I get ********** on the extension.