Brand New SIM

• The iPhone 4 has a tray that holds its SIM card. The tray is easily popped out from the side of your phone, which gives you access to the SIM. If your current card is damaged and needs replacement, swapping out the current card with a new one takes just a few seconds. You'll have to reactivate your phone, however, using the iTunes activation process or with the help of a representative from your cellular service provider.
  
  

Other Phone SIMs

• Most other SIM cards won't fit in the iPhone 4, as it requires a micro SIM. Most other GSM cellphones use a mini SIM, which won't fit properly in the card tray for an iPhone. If you have a micro SIM card available from another iPhone, it will fit into your phone, but you'll need to activate your phone again with the new SIM card. Older iPhones use a mini SIM card and can accept SIMs from other GSM cellphones.
  
  

SIM Card Information

• Many cellphones use their SIM cards to save contact information. The iPhone doesn't, instead saving contact information to the device's hard drive itself. If you replace your current SIM card with a new one from another iPhone, you won't be able to get contact names and numbers from the card. If you have an older iPhone that accepts mini SIM cards, the previous phone may have saved contact info to the card. In that case, use the "Settings" menu on your phone to download the information on the card to the iPhone.
  
  

Replacing the SIM

• Some iPhone models come with a SIM ejection tool, but if you don't have one, an unraveled paper clip will also work. Power down the phone and stick the tool or paperclip into the hole on the SIM card tray. When you press your tool firmly into the phone the tray pops out. Lift out the current SIM card and lay down the new one with the metallic part facing down. Push the tray back into the phone.