Tax administration of Turkey provides more easy way to pay for phone registration if you bring it from abroad – you can register its IMEI code online:
You can use that service if you have your Ikamet already, as online payment will require your TC kimlik no. (its written on Ikamet card).
Also you will need to fill in your full name and last name (as it says in ikamet) and IMEI number of your phone. Registration fee is: 131,50 TL. You will pay by credit card.
If you still can not pay online, you can do that at any Tax office (vergi dairesi). Dont forget: you should do that during first 60 days after you crossed the border. Otherwise phone will be blocked when you start to use Turkish simcard without IMEI registration. If you just use your foreign simcard – registration is not required.
Please, note: you can register only 1 phone for 1 person during 2 years. You can read details here:
Be aware that if you buy Turkish phone, new or used, you can just use it directly with Turkish simcard without any extra registration required.