Still overbilled, 3 months later

I wrote a complaint about being over billed 3 months ago, still it hasn't been resolved. I phoned the call centre last month to complain and my account was credited, again this month I'm being over billed. Why autopage, was this not corrected last month? or in January for that matter!?! I don't have time to waste phoning every month. Your service used to be good, now it's pathetic!