The majority of cell phones are designed with SIM (Subscriber Identification Module) cards.  These cards are microchips that hold subscriber data.  The card provides cell phone service by activating the phone in which it is inserted.  Locked phones will only recognize SIM cards provided by the carrier.  On the other hand, unlocked cell phones will work with SIM cards from any carrier.  Locked cell phones are used to keep customers loyal to one particular company.

Cell phones can be unlocked by entering special codes.  The process, once completed, is usually permanent.  Certain companies have begun to offer unlocking codes and processes.  Normally, an individual will unlock their phone with their IMEI (International Mobile Equipment Identity) number.  The company who received the information will process it and e-mail an unlocking code with instructions.  Once the users inputs the code, the phone is unlocked.  Users who wish to unlock their phone must be careful because there are several fake random number distributors.  Unlocking can also occur by the user sending their phone to the vendor for physical unlocking.

iPhones can be unlocked, as well.  Apple iPhones are locked or unlocked when the device is activated.  During activation, the SIM is locked or cleared depending on the information within the IMEI.

In the United States, T-Mobile will unlock a cell phone if the user has an active account and good standing within the last 90 days.  The phone may also be unlocked by T-Mobile if the user pays full retail price and provide proof of purchase.  AT&T Wireless will normally unlock a cell phone after 90 days or once a user has concluded their contract.  Carriers are not obligated to unlock cell phones by law or any regulations.  The companies can refuse to unlock a cell phone.  AT&T, for instance, will not unlock iPhones at all, even after a contract has expired.

Since they are not attached to one company, unlocked cell phones are able to be used worldwide.  Therefore, for world travelers, unlocked phones prove to be quite useful.  The user can remove a SIM card from one carrier and replace it with the next.

For cell phones to work outside of the United States, they need the proper amount of bandwidth.  Phones, such as Tri-Band, have been documented to work efficiently out of the country.  Tri-Band offers 800, 900 and 1900BW for maximum use and advantages.  Conducting research and determining the best carrier is the only way to get good service.