Phones

I don't know why buying a "cheap" phone is more expensive in Turkey than in most of Europe, but Rick's Istanbul guidebook indicates that it is. Page 409 suggests you'll pay $80 or more for an unlocked phone with Turkish SIM (versus under $50 in other countries). If you have an unlocked phone that will accept a new SIM card, that's a cheaper purchase; see general tips at http://www.ricksteves.com/travel-tips/phones-tech.

I'm not addressing your question directly but I signed up with TMobile where there international rate is $.20 per minute with no contract. They have a LG smart phone for $50. The monthly charge is around $60 and I could discontinue when I return. I'm going to try that next week in Turkey on my RS tour.

We recently used Viber app. on our phone and it worked great especially for texts...You can get temp phone easily at airport but we brought our own...

Your phone from the US or Europe will work for 3 weeks before it is turned off.

if you have phone from out of Turkey, yes it is going to closed by government in 2 months not in 3 weeks!
You can buy Turkcell or Vodofone prepaid plans. I always do that wherever I go to another country, I buy their local prepaid card. Its really helpful. You will not pay more than $30 USD for 4GB internet data and 1000 minutes talking time.
Its easy to get.Just show them your passport at Turkcell or at Vodofone,( they have Turkcell and Vodofone kiosks in Ataturk and Sabiha Gokcen Airport ) and get it in 10 min.