Over billing

I took a cellphone contract a few years ago when this company was still new. No issues and I paid 12 months hustles free. Decided to get another contract which was 12 months. I Paid from Jan to Dec after Dec I was informed to pay 2 months for ownership. I PAID 3MONTHS AND NOW I HAVE BEEN INFORMED I OWE 3 MONTHS. 15 MONTHS WORTH OF PAYMENT. WHAT WAS IT FOR CAN A PROPER PERDON TELL ME WHAT IS GOING ON WIRH RENTOZA?