Foreign visitor may enter Turkey with one personal mobile device and use it for up to 120 days via international roaming or with a Turkish SIM card without having to pay Customs duty, or to register it. After 120 days, you must register your device and pay the registration fee or your device will be blocked from the three Turkish mobile phone networks.
Turkcell: coverage is the best in Turkey, pretty much everywhere, except for occasional relatively small dead spots in a few remote and/or mountainous areas in the eastern region of the country
Vodaphone: with 22% of the mobile phone customers in Turkey, has fairly good coverage in towns and cities larger than 10,000 population, but the least complete coverage of the three companies