SIM Card Size

• Not all phones require the same size SIM card, even among iPhone models. The iPhone 4 and newer models use the micro SIM, which at the time of publication is rarely found in other devices. The iPhone 3GS and earlier models will accept a mini SIM, which is used in other phones more frequently, so the chances of your SIM fitting in an older iPhone are better. If you attempt to use a wrong-sized SIM card in your iPhone, it will not function and you won't be able to activate the phone.
  
  

Cellular Carriers

• Not all cellular carriers are equipped to offer iPhone service. Unless you have unlocked your iPhone to work on any network, putting a SIM card from one carrier into an iPhone from another carrier will not work. As long as your SIM card is from the same cell carrier that originally sold the iPhone, however, the SIM card will be able to activate in the new phone.
  
  

Activation Process

• When you first put your SIM card into an iPhone, the phone will not work right away. An iPhone that has already been activated is programmed to work with its original SIM card. In order to activate the new SIM card with the phone, you must attach your phone to a computer with iTunes. Upon opening iTunes, the iPhone will be recognized and iTunes will automatically begin the activation process.
  
  

SIM Information

• If your SIM card has contact information saved to it, your contact names and numbers can be uploaded to your new iPhone. The "Settings" menu on your iPhone has a "Mail, Contacts, Calendars" subcategory, under which you will see an "Import SIM Contacts" icon. Tap the icon to add the SIM information to your iPhone "Contacts" application. However, iPhones do not store contact information to a SIM, so any new contacts you add to your phone won't be saved to the SIM.