How the hell do you get billing mistakes fixed?

I was abruptly over billed two months ago. I tried working with billing, but was only allowed to leave messages, and to date, two months later, have not received a single response to my message. I worked with an account manager and sales person to make sure my account was set up correctly and establish what we should be charged each month. Yesterday for the third month in a row, I was over-billed again. How do you get to someone that can actually fix this.