If you’ve bought a phone from outside of Turkey, or want to continue using your (non-Turkish) mobile in Turkey, you can’t just put a Turkish SIM card in it and expect it to work. Even if you get away with it for a short time, pretty soon your phone will be detected and shut off by the Turkish authorities.
So, here’s what you have to do. After you arrive in Turkey, you have 30 days to register your foreign mobile with the telecommunication ministry so that it can work in Turkey.
To do this, you need to head to the Vergi Dairesi (‘tax office’) with your passport and mobile phone. You will also need a tax number, which is very simple to obtain from any tax office. When you get there, they will ask for your phone’s IMEI code (click here to learn how to find it), and ask for a registration fee / tax of 115TL.
There are several tax offices in Istanbul where you can do this, but a fairly central one is in Şişli near the police station (see map below). The nearest metro station is Gayrettepe, and from there the tax office is a 15-minute walk.
Once you have registered your phone, you will be given a receipt which you then need to take to your preferred phone shop (e.g. Turkcell or Vodafone), together with your passport. They will be able to register your phone (which costs a further 20TL). You’re then good to go!