There are three mobile operators in Turkiye: Turkcell, Vodafone, and Türk Telekom.
Turkcell has the greatest coverage and most customers, and is the best option for most visitors to Turkiye.
Vodafone sits in second place in terms of both coverage and subscribers. It’s a reasonable alternative if you need vast amounts of data, as it offers larger prepaid data package options than Turkcell.
Turk Telekom is the smallest operator, with the least coverage outside the major towns and cities. There’s no particular reason for travelers to go with it, unless you plan to only visit larger centres and happen to come across a particularly good price.
Note that all foreign phones will stop working 120 days after they are first used in Turkey, unless you visit a tax office and pay a significant fee to register them. This won’t affect the vast majority of short-term visitors, but is something to bear in mind if you plan to return with the same device in the future.
It’s technically the IMEI that gets blocked, which is allocated per SIM card slot. If you have a dual-SIM phone, you can get around the problem by switching your SIM to the other slot after 120 days. You’ll otherwise need to use a different phone, or buy and use an unlocked mobile hotspot, on your next trip.
All three operators have stores in the arrivals lounge at both of Istanbul’s airports (the new Istanbul Airport, and the more distant Sabiha Gocken) so you don’t have to even set foot in the country without coverage.
Even outside the airport, you may find vendors are willing to sell you any prepaid SIM packs.
You will need to show your passport to sign up for any plan. The salesperson will fill out the paperwork for you on the spot.