SIM cards are always locked to their carriers.
By default, SIM-bearing cell phones in the United States are sold "locked" to their original carriers. In other words, you can only use a cell phone with a subscriber identity module or "SIM" card issued by the carrier who sold you the phone. A phone can be locked or unlocked, but the SIM card is always "locked" to the carrier who sold it, so re-locking a phone you've previously unlocked has no impact on the locked status of the SIM.
Unlocking the Phone
The procedure you follow for unlocking a cell phone depends on which carrier you use. Certain carriers offer "unlock" codes for devices once you have completed a certain contract period, such as AT&T with the BlackBerry. In other instances, you can download software onto the phone and unlock it yourself. Alternatively, take the cell phone to a local repair specialist and have him unlock the phone for you. Once you unlock a phone, you can insert any activated SIM -- SIM cards are always "locked" -- into it.
Is My Phone Locked or Unlocked?
Before locking a phone, make sure that the phone isn't already locked. To see whether or not a phone is locked, insert a SIM card from a carrier other than the carrier from whom you purchased the device into it -- borrow a SIM from a friend if you don't have a spare lying around. If the other carrier's name appears in the upper, left corner of the phone screen, it is unlocked. To re-lock your phone, update the phone's software next time the computer prompts you to do so.
Reasons to Re-Lock Your Phone
Unlocking a cell phone gives you the freedom to use the device with any carrier's network, but unlocking isn't without risk. First and foremost, not all carrier networks support all devices, so using a cell phone on a network that doesn't support it leaves you alone in the troubleshooting process if the device experiences errors. Furthermore, unlocking can damage a cell phone's hardware and software and in many instances -- in the case of the Apple iPhone, for example -- unlocking voids the device warranty.
Obtaining and Activating a New SIM
If you re-lock your phone to its original network but don't have a SIM from that carrier, you need not only to obtain a new carrier from the carrier -- you can purchase these in a carrier retail store or online -- but also to activate service to the SIM card. SIM cards serve merely to allow a cellphone to connect a carrier's network, but don't allow you to make calls, send texts and use the Internet on your cell phone in and of themselves.
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