The major GSM operators in Turkey are Turkcell, Vodafone and Avea. You can use your mobile phone in Turkey if your provider has enabled international roaming. However if you intend to stay for a long time in the country or make several calls, it may be preferable to buy a local prepaid SIM card. Take your mobile phone and passport to a Turkish mobile phone shop where your new SIM will be registered along with your handset’s IMEI number and your personal information. (Unregistered phones will be blocked and unable to receive or make calls.) Turkey has very wide mobile coverage networks so you shouldn’t have any problems in the main cities and tourist resorts.