The iPhone is a quad-band mobile phone based on the Global Systems for Mobile Communications (GSM) network. Phones operating using the GSM standard have Subscriber Identity Modules (SIM) cards. SIM cards allow cellular service providers to identify cellular subscribers using their network. SIM cards are removable and a cellular subscriber can transfer a SIM card between phones. The iPhone is normally sold with a carrier lock that prevents using the phone with another cellular service provider's SIM card. You can tell if your iPhone is locked, by inserting another SIM card into your iPhone.
Power off your iPhone.
Unfold the end of a small paper clip so one of the ends is exposed.
Insert the paper clip into the small hole located at the top of the iPhone. The hole is next to the headphone jack at the top of the iPhone. The SIM card tray will automatically pop out after you insert the paper clip into the hole.
Pull the SIM card tray out of the iPhone.
Remove the existing SIM card from the SIM card tray and place the new SIM card in the SIM card tray.
Push the SIM card tray into the empty slot located at the top of the iPhone.
Power on your iPhone.
Look at the upper left corner of your iPhone's screen. If your iPhone says "Invalid SIM," your iPhone is locked and will not work with the new SIM card. If, on the other hand, your iPhone shows the logo for the cellular service provider associated with the new SIM card, your iPhone is unlocked and will work with the new SIM card.