Mistaken Bill

I joined VM on 17 Jan 2024 and the monthly charge is  £26.99 according to the contract. However, I was charged for £75.98 for the first bill. Instead, it should be £53.98. I called the helpline for 5 times and sent 2 emails. Your team has replied me with an email to confirm there will be one-off credit of £20 and recurring credit of £12 for the rest of 16 months of my contract, but I have not received my revised bill still and they have closed the case. I do not understand why it takes so much time to rectify the bill when you can cross check with the contract. I regret to join VM!