Refund and credit bureau status

I was over billed then decided to reverse the debit order n pay what i could afford became frd up. N decided to pall the balnce reflecting in my statement only to find out later that inwas over billed ans now im struggling to get a refund frim. Cellc ive sent emails and called call centers still nothing on top of that it shows that i owe cellc for months n its affecting my credit score