Hopefully someone with more experience in Turkey will answer but if not, here's our experience. We were in Turkey last September and two years earlier had used our US phone and a Turkish SIM card. Last year we learned there were new restrictions and all cell phones need to be registered with the Turkish government. Initially unaware of this we purchased a SIM card as we had done previously. It never worked but as we were on a driving trip we were unable to return to the original shop. Long story short, we ended up buying a new, inexpensive phone there. It works in all of Europe but it is dual band only so it does not work in the US. The sad part is that we already had our own unlocked quad band phone that worked in Europe. I'm not aware of this restriction in any other countries.
If you were not going to Turkey, I would suggest to you what we did a few years ago. We bought an inexpensive used, unlocked quad band phone on Amazon. We then bought a prepaid US SIM card and used the phone here to be sure it actually worked. Since then we have used that phone in Portugal, Greece, France, and Italy by buying a local SIM card, with no problem. Since you are going to Turkey you might try to get more info on the phone restrictions. Trip Advisor might have more people with recent experience with cell phones in Turkey. Good luck!