I brought my factory unlocked iPhone 5 with me and bought an Orange prepaid SIM card in Madrid airport, all they needed to see was my passport, costs 15 euros with 1GB of data for a month, I had to add an additional 5 euros for voice/text credit. You don't have to "sign up" for any carrier if you buy the unlocked version directly from the Apple store, I use T-Mobile in the US with a no contract SIM card, I am able to switch from my US SIM card to my prepaid TELCEL (Mexico) SIM card when I travel down there as well, I have used 3 different SIM cards since I bought my iPhone 5 and all have worked with no problems and no crazy data bills when I get home. PM me if you have any questions.