Vodacom shocking service and billing issues.

I have been struggling with Vodacom for months now to sort out by billing. First they overcharged me for a handset, which I tried to fix for 3 months. They still went ahead and deducted it from my account. I reversed the payment as I told them I would do. They subsequently disabled the debit order and I have been trying for last 3 months to reinstate debit order. They keep on sending messages that account is in arrears and I have phoned their support desk twice to try and reinstate it. Now when I want to upgrade they tell me my account is in arrears. What do I need to do to get them to take the money I owe them. Is it that difficult.