Summarize this article:

Usually, if you've been a customer of theirs for some time, they will provide you with an unlock code for free.  This is, by far, the best way to get your phone unlocked. After you connect with your provider follow what they say to unlock. To determine how to remove the SIM card of your particular model, reference your user manual. Enter your PIN number if you are prompted. For newer models, simply insert a new SIM card and enter your unlock code. You can find this by using easily downloaded software. If you have unlocked your device correctly, you will see "SIM restriction off" on your screen. If you are using an older model, proceed to the next step. # PW + unlock code + 7#. Enter the P by tapping the * three times. Enter the W  by tapping the * four times. Enter the + by tapping the * twice. If that code doesn't work, try replacing the "7" in the code with a "1." If you have unlocked your phone correctly, then "SIM restriction off" will appear on your screen.

Summary:
Contact your service provider. Turn on your phone with no SIM card inside. Key in the following code: Unlock your Nokia device.