MOBILE PHONE USE IN TURKEY
Temporarily Staying in the Country (less than a month)
People who will stay in Turkey for a temporary period (less than a month), shall make subscription agreement with one of the GSM operators in Turkey; then IMEI number of the equipment which is owned by those people is matched with the MSISDN number, and the related IMEI number is opened to communications on condition that only matched MSISDN number is used with this equipment.
In this context, after making application to subscription centers of related GSM operator with a passport indicating that person has entered Turkey up to a month before, MSISDN number and related IMEI number is matched and the device is opened to communications. The price for a line is about 10 EURO.
You can see more in here: https://www.mcks.gov.tr/en/KonuDetay.php?BKey=45
Residents and long-term visitors
Foreign residents are entitled to bring one mobile phone into Turkey within two calendar years for use during their stay in Turkey. The phone must be registered within one month of arrival to Turkey. You shall pay a tax (115 TL) when registering your mobile phone.
It is necessary to register the mobile phones in order to use them with SIM card bought from a Turkish network operator. (In order to use such a mobile phone with a SIM card bought in Turkey from a Turkish network operator, the mobile phone number assigned to the SIM card needs to be correlated with the IMEI number of the mobile phone. The handset can only be registered with one line. Phones not registered in this way will be blocked and unable to receive or make calls.)
No customs documents are required for the registration of mobile phones.
Mobile phones to be brought into Turkey are to be registered with the mobile phone shops of Turkish Network Operators (Avea, Turkcell or Vodafone).
Vodafone : https://roamingturkey.com/
Avea : https://www.avea.com.tr/web/en
Documents required for the registration of mobile phones are:
passport (Identity details indicated on the passport)
copies of all Turkish entry/exit stamps in the passport
No more than one mobile phone per person can be registered in two calendar years.