Turkcell Reduced My 12-Month Contract to 11 Months and Increased My Bill

Hello, I have been a Turkcell customer for years, but I am very disappointed. Two days before my old plan ended, I signed a new 12-month contract. However, when I checked, I realized it was only for 11 months. I called customer service, but they said there was no mistake. I even double-checked my contract details, but nothing explained the missing month. Not only was one month of my previous plan taken away, but my bill is also higher than expected. How does this calculation make sense? I request that this mistake be corrected. Otherwise, I will end my relationship with Turkcell. If customer satisfaction is truly important, I must say I am not satisfied.