Turkcell Charged Me ₺613 After My Contract Ended Despite Promising Only ₺97

I used to have a Turkcell phone line, but I recently stopped my contract because the time I was supposed to keep it was up. When I called customer service twice, they both told me I would only be charged for the time I used. At first, the balance showed ₺97, which seemed fair. When the bill came, though, I was shocked to see that it was for ₺613. My contract had already finished, so there should be no fee to get out of it. I don't get how Turkcell can send a bill for such a large amount of money after the contract is over. I want these charges to be cleared up right away and for more information.