Buying a Turkisch SIM card for your mobile phone you brought from home can come with some unexpected intricacies:
First, as for any country your phone should be unlocked for foreign SIM cards, that is it should not be restricted to your home provider. If your phone isn´t or you are not sure about it, you can have it unlocked in a mobile phone shop.
Now for your unlocked phone you can buy a SIM card in Turkey for example from Turkcell or Vodafone. These are major providers with numerous shops even in small towns all over the country. I paid 50 Turkish Lira (ca. 15 €) for a Turkcell SIM card which contained credit for 30 text messages and 60 minutes of national or international calls. This credit was, however, valid for one month only after which I could either stock up on credit or be left with a SIM card which can only receive texts and calls.
Nothing spectacular so far. I had been vaguely warned, however, about some special registration issues for SIM cards in Turkey. When at purchasing the SIM card I had to present my passport of which a copy remained in the Turkcell shop, gave them an address (of a friend) in Turkey and made three signatures – all for a ridiculous pre-paid SIM card – I felt fairly confident that this must have been what people were talking about. What else could there be?
After 10 days of happily using the SIM card, my phone suddenly screened „SIM not registered“. After running to 4 Turkcell shops in Istanbul (all within 2 km of each other…), I realized that I actually had to pay some sort of custom fee on my phone if I wanted to use it in Turkey. I was told this tax was as high as 120 Turkish Lira (ca. 35 €) which was almost twice the price my cheapo mobile phone had cost three years ago. In the end I therefore decided to buy a new phone for 100 Lira for which no custom fee had to be paid, because it was sold in Turkey. After three weeks of using this phone there have been no further problems with the SIM card…
In a nutshell: If you go to Turkey and want to use a local SIM card during your stay, be prepared to either buy a new mobile phone in Turkey or to pay 120 Lira customs fee to be able to use the phone you brought.