Overcharging

My contract got into arrears and got suspended. After settling they reopened my account but started overcharging my data line. Been trying to get help for over a month and nobody can help. The one guy from accounts who came close to understanding me said he'll investigate and get back to me, and it's over a week and am still waiting. I've decided to pay less for what I get charged from now onwards. I'll subtract whatever they've added and also find a way to get back what they already owe me bcos this is soo not on.