The SIM card or Subscriber Identity Module is a removable memory card inside your iPhone that stores information identifying the device on a cellular network. Follow these steps to remove or replace the SIM card on your iPhone:
1. Locate the SIM tray. On the original iPhone, iPhone 3G and iPhone 3GS the SIM tray is at the top of the device. On the iPhone 4 and later the Micro or Nano SIM tray is located in the center of the right side.