Billed for 12 month Mobile free promo plan

I signed up for the free Mobile plan for 12 months promo back in September. Now I see I'm being billed again. I have spoken to customer service about this last month and the month before, do I have to contact someone every month? This is very frustrating. Every month I have to call customer service to try to get this fixed. They credit me and they say they will fix it so I won't be billed again for the rest of my promotional period. But it never gets fixed. How do I fix this?