In the United States most iPhones are locked so that they will not work unless the user purchases a contract with a particular provider. There are some ways to unlock an iPhone and recently the government stated that it is not illegal to unlock your personal phone. When a new phone is purchased, the first thing asked by most users is how can I tell if my iPHone is unlocked? It is a good idea to check out a used iPhone before purchase if possible to see if it is locked or not.
To see if an iPhone is unlocked first turn it off and then remove the SIM card from the slot that is located at the top of the iPhone. Once the SIM card is removed replace it with a new SIM card and then close the phone. After you have replaced the SIM card turn the phone on and wait for at least 2 minutes for the phone to reset. The iPhone needs time to sync with the new card and find the proper associated network. Then try to make a call on your iPhone. If you can make a call successfully then the phone is unlocked.