No refund for incorrectly charged account

I upgraded my phone contract last year October and signed over to a new package as well. I have 2 phones on the one contract. I couldn't understand why my account was suddenly so high and thought that it must be because of the new package with new data charges, but then almost 2 months ago i went to query the high cost, only to find out that they migrated only 1 of the phones and not both. I questioned this as it was 1 contract and not 2. They advised they can only credit me 3 months back, but it would be credited to my next account statement. I was not happy with that, so i contacted head office. They logged the complaint and told me to call back in a week. I did and they told me there is nothing to credit. I logged another complaint, called back a week later and they gave me an amount to be credited to my next account. I insisted that this money should be paid into my bank account as this is money incorrectly paid to them due to their negligence. They logged another ticket. Again i phoned a week later to be told 'there is nothing to credit'. I asked to speak to a manager, was put through to 3 different depts before the line dropped. All i want is service and my money!