Turkcell Charged Me an Extra Fee Without Explanation After My Contract Ended

I had been a long-time Turkcell user, but after my contract expired, I switched my mobile operator from Turkcell to Vodafone. I always paid my bills on time and expected a smooth transition after the contract ended. Unfortunately, at the end of my contract, Turkcell added an extra charge to my final bill, claiming it was for annual internet usage. This charge was never disclosed in advance, which further proves its illegitimacy. I raised my complaint through consumer complaint websites, but Turkcell has refused to address the issue. This additional charge is completely illegal, as I did not use their services. I demand a refund of this payment as soon as possible.