Upvote:2
You have two options:
Option 1 requires a visit a local phone carrier store. You will swap out your sim card with a local one.
Option 2 requires purchasing online, no more visiting a store, no more swapping sim cards physically and you still get to avoid unexpected roaming charges. Order online, receive a code, change your settings and you are connected to a Turkish mobile network.
I personally use Vodafone in most countries I visit. Vodafone has tourist packages in Turkey, allowing you to keep a local number for 3 months. I hope that helps.