Prompts lock code when sim is changed - Nokia 2600

If your phone uses a SIM card to operate, you can activate a security feature called a SIM PIN. The SIM PIN protects your SIM card from unauthorized use, and must be entered in order to use your phone. Enter it incorrectly three times and your SIM card becomes locked. You can unlock your SIM card on your own, however, by typing in a PIN unlock key. Your carrier can provide you with the unique PUK code for your device.

1 Contact your wireless carrier by calling the toll-free customer service number. (Find this on your provider's website or a recent invoice.) You will need to verify your account password before the PUK code is released to you.
2 Power your phone on. You will see a "SIM Locked" or "Enter PUK" message. Type in the eight-digit PUK code and press "OK."
3 Type in a new four- to eight-digit PIN code when prompted, then press "OK."
4 Verify your new PIN code by re-entering it when prompted. Press "OK." Your SIM is now unlocked.

1. Browse to your carrier's website and enter your account login, or follow the prompts to register. This will include entering details such as your telephone number, name and a password. The password on the website account is unrelated to the SIM card password.
2. Select "My Phone" or "About Phone" or something similar and look for a settings summary.
3. Enter the PUK code from the website on the phone when prompted to do so. The phone will prompt you after three incorrect PIN number attempts. You will have 10 attempts to enter the correct PUK code before your SIM card locks up and becomes unusable.
4. Enter a new four- to eight-digit PIN code when prompted to do so. The phone will prompt you after you have successfully entered the PUK code.
5. Re-enter the desired PIN code to confirm.

Im assuming its a samsung phone
Samsung cell phones come with many security features.
In addition to the common feature of preventing calls after a period of inactivity, some Samsung phones also allow you to prevent others' use of your phone's SIM chip and access to your files.
One password unlocks the various locks, except for two other passwords used for the SIM chip. The syntax of the menu items to click to reset a password differ somewhat between Samsung models but you can figure out roughly how to apply instructions for one phone to another.

1
Lock the phone from making calls by clicking "Start" or "Menu," "Security Settings" (or "More" and then "Security"), and "Phone Lock" or "Device Lock."
Change the password by clicking "Start" or "Menu," "Security Settings," and "Change Password." Then enter a new password.

2
Set the phone to work only with the current SIM chip by clicking "Menu," "Security Settings," and "SIM Lock."
Enter a new password when prompted.

3
Prevent unauthorized use of your SIM card by clicking "Menu," "Security Settings," and "PIN Lock." Type a new PIN when prompted.
4
Prohibit access to your files, messages and contact list by clicking "Menu," "Security Settings," and "Privacy Lock."
Enter the password from Step 1 when prompted to unlock access.
Reset the file lock/messages/contact password by resetting the phone lock password, as in
Step 1

1) Contact your service provider to obtain instructions and the SIM unlock code for your device.
2) Download one of the following files:-
a) sim ulock tool serial blackberry.zip
b) sim unlock tool usb blackberry.zip
3) Extract the file and save it to your personal computer.
4) Connect the device to the personal computer.
5) Double-click the downloaded file. If prompted, enter your device password.
The unlock tool runs, and the device resets when complete.
6) Proceed with the SIM unlock instructions from your service provider.

NOTE:-This happen's when you're trying to change the service provider.
If you intend to change your service provider, contact your current provider and inquire whether your BlackBerry device's SIM card is locked. If your BlackBerry device has a locked SIM card, you will need an unlock code. Some carriers charge a fee for this. Once the BlackBerry device is unlocked, it will accept a new SIM card and a new service provider.

If its the phone lock code and you have forgotten it, only Sony Ericsson will be able to unlock the phones lock code. Have you tried the default code which is usually either '00000' or '00000000'? If you have changed it from original default, unfortunatley there is no master code available! It has to done by manufacturer.
If its your SIM pin that is unknown then make sure you have your mobile number and call your network provider for a PUK code. Once you have this enter any code and after 3 attempts it will lock sim and request PUK code. Enter this and it will prompt you to set new pin, and repeat to confirm. Done.

you may have locked your sym card. if you did, your phone would prompt you for a password when it turns on, just to access the network. try some basic codes like 0000 or 1234. these are most likely the factory defaults.

Securing your Shine
Secure your LG Shine and your account from thieves by locking your phone and your SIM card Losing a phone through accident or to a light-fingered thief is annoying, frustrating and potentially very costly, especially if the 'finder' uses your SIM card to make expensive calls. You can, however, lock both your phone and your SIM card with four-figure PIN numbers to prevent unauthorised use.
The first line of defence is to set a SIM card PIN. This means that whenever you turn on your handset you have to enter a PIN code before the SIM can be used. The default PIN code for your SIM is dependent upon your network; usually it will be found in a small booklet that came with the SIM card.
The PIN can be enabled, disabled or changed from the Security menu – but you only get three chances to enter it correctly. After those three, the SIM will get locked and can only be unlocked by entering the PUK (PIN Unlocking Key) which your network can give you. Enter the PUK incorrectly 10 times, and the SIM will be irrecoverably locked.
To activate a PIN code request, open the main menu by clicking on the scroll wheel. Click 9 to access the Settings menu and click 6 to enter the Security menu. The first item is PIN code request - click 1 or OK to open it. By default this will be set to off. To activate a PIN request, click 1; you'll now be prompted to enter the four-digit PIN. When you've done this, click OK, and if the pin was correct, you'll see the confirmation message 'Complete'.
To deactivate the PIN code request, you would simply select Off from the menu by pressing 2; you'll need to enter the PIN code whenever you change this setting.
PIN code request prevents another user from using your lost or stolen SIM card in their own phone. To lock your phone, and thus render it useless to a thief, you can activate Phone lock.
Two levels of handset locking are available. The first - called 'When power on' in the Phone lock menu - is to require an unlock code to be entered whenever the handset is switched on. This can be confusing and irritating when used together with the SIM PIN, so a second option is to only request the handset unlock code when the SIM card has been changed - called 'When SIM changed'. This means that if your phone is stolen, the thief can't use their own SIM card in it as the phone will need a PIN code to unlock.
To activate either option, go to the Security menu (clicking 9 for Settings and 6 for Security from the main menu) and click 2 to enter the Phone lock menu. Click 2 to activate When power on or 3 to activate When SIM changed. In either case you'll be prompted to enter the security code. By default this is 0000. Enter the code and click OK to confirm.
To change either code, go to the Security menu again (clicking 9 for Settings and 6 for Security from the main menu) and click 5 to enter the Change codes menu. To change the handset's security code, click 1 to select Security code, or to change the SIM PIN, click 2 to select PIN1 code. In either case you'll be prompted to enter the old code before entering the new code twice, clicking OK to confirm each time.

Hi! I am locked out of my phone & it is prompting me for the PUK code and I can't find this?