Paying an extra month

Hi

My 36 month contract is up at the end of December, I called the call center to make sure and to make sure i dont get charged for the service option. They then however told me that my final payment would be in January as a "Handover fee" This would then make my contract 37 months.

I didnt sign up for 37 months. I would like to know where this is in the contract.

Thanks