Can I use my mobile phone when in Turkey?
There are 2 answers to this question:
You can use your mobile phone without any limitations in Turkey if you keep your original number (and/or SİM card) in it.
You can still use your mobile phone in Turkey with a local number (or SİM Card) however your number will be blocked within a few days if you do not register your mobile phone with the Tax Office. In order to keep using your local number without limitation, you need to go to the nearest tax office and pay 80 TL (as of 2015) and register your phone with authorities.