For a GSM (Global System for Mobile Communications) iPhone 4, a SIM (Subscriber Identity Module) card is necessary. This tiny memory chip contains information such as contacts and security settings and the wireless carrier. A GSM iPhone 4 can be used anywhere in the world as long as you have the SIM card; however if a lock message appears on the screen, you can unlock it and gain access to your phone's features.
Unlocking the SIM Card
If the "SIM Locked" message appears on the screen, it usually means the SIM lock has been enabled. iPhone 4 allows you to lock the phone's SIM card if you don't want other people using the phone without your consent. When the lock is enabled, you have to enter your Personal Identification Number (PIN) to turn on the phone and access the its features. To unlock the phone, tapping the "Unlock" button on the screen will bring up the PIN entry screen. From there, entering your PIN number will display the home screen.
SIM Lock Settings
You can enable or disable the SIM lock security setting anytime you want. In addition, you can change the access PIN number. This may prove useful if you suspect someone knows your current PIN. To change the SIM lock settings, tapping "Settings" on the home screen will bring up the phone's settings. From there, tapping the "Phone" option and tapping "SIM PIN" will allow to disable or enable the feature and change the PIN. Once changed, the settings will take immediate effect.
Tips and Recommendations
Some wireless carriers provide a default SIM PIN that can be used until you decide to to change it. If you're not sure what your default SIM PIN code is, check the phone's documentation. Contact your wireless carrier if you can't find the SIM PIN code information.
Warnings
If you enter the incorrect pin number more than three times, the phone is automatically locked. According to Apple, when the phone locks, you must enter a Personal Unlocking Key (PUK). The key varies with different SIM cards, so refer to your phone's documentation for this information. Like the PIN, ff you enter the an incorrect PUK code several times, the SIM card will lock automatically. This permanent lock is irreversible, so contact your wireless provider for a replacement card.
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