If your trip to Turkey lasts only a week or two, you probably don't have to register your mobile phone, but if you plan to stay several months, especially if you have a residence permit (İkamet Tezkeresi) you will probably have to register your phone and pay TL115 in tax, or purchase a mobile phone from a shop in Turkey (thus also paying the tax).

http://www.turkeytravelplanner.com/details/Communications/registration.html

Calls are tracked based on usage units (kontör). The cost of a call depends upon your usage plan (there are many), and what you paid for your usage units, but ranges from about TL0.30 to TL1.80 per minute (about US$0.23 to US$1.35, or EURO0.19 to EURO1.14).

You can order prepaid SIM cards from various companies and have them delivered to you before you depart for Turkey, or you can go to a mobile phone shop in Turkey and buy a starter packet.

http://www.turkeytravelplanner.com/details/Communications/prepaid_plans.html