Turkcell Charged Me for an Unknown Service, Tyg Invoice

Every month or so, I receive these strange invoices from Turkcell with the subject line "tyg." I was taken aback when I opened this month's bill and saw a staggering ₺2640! I am currently stuck overseas, so you can imagine my surprise! I'm not using the line, no idea where this is coming from, is somebody else is using it???  What on earth is that? They're attempting to swindle me to death. Even worse, I am completely at a loss as to what to do, not sure if they'll take the charges back. I'm terrified that Turkcell will try to avoid my call and instead charge me more for something I didn't agree to when I signed up.  terrifying!