It depends on the model of the iPhone. Some iPhone have sim and some doent. The size of the sim depends on the model too. You can check for the sim qualities before you buy. If you are buying a locked sim you need to stick to that sim. If you want to change the sim, then you need to unlock. To unlock, I suggest you can approach Mobileunlocksolutions.com and specify the country,locked network of iPhone.Based on the availability you can get the permanent unlock service and for free unlock instructions visit Mobileunlockguide.com