MalikaGupta86, There is excellent network all over Turkey, 99% of areas. However if you use your own T-mobile operator, you will be paying hefty charges because signal will first be routed through US. I suggest you buy a Turkish sim card at the Istanbul airport from one of the following mobile operators and get a package. Turkcell, Vodafone or Turk Telekom.
Thank you enigma2007.. Could you please let us know the likely pricing?
I realize this is an old post, but for anyone stumbling across in the future...
Reply #1 is incorrect.
There would be no extra charge for text/data with T-Mobile in Turkey, if you have a post-paid plan (the normal plans many have like T-Mobile One or Simple). T-Mobile has for years, included free international text/data for the vast majority of international locations (140 countries). Voice is $0.20/minute.
I have used T-Mobile in many countries and it is generally quite serviceable; you aren't going to get the speed you are used to at home, but works fine for e-mail, light surfing, navigation, etc. Texting works very well, whether to someone else local or back to U.S.
Countries covered for free: