Billed incorrectly for two months

My contract ended last year on the 11th and degraded my contract and was told they going to take the full amount and credit back a specific amount in January. I've been calling their help line for the past week and can't seem to get help regarding this matter. They billed me incorrectly last month,this month and wasn't informed about this. Why must you take my money in my account without letting me know but it's a drag to credit it back.