There are two ways to do a factory reset on your Nokia 6600
The soft-format code for Series 60 phones is *#7370# . Enter format code in dial screen. It performs a format of the Internal drive - All data will be lost ! Default security code is 12345
The method of performing a Nokia 6600 hard-format using key
1. Switch off the phone.
2. Hold down the following three buttons: Green (call answer) button, star * button, and '3' button
3. While holding these buttons, press the power button and switch on the phone
4. When the message 'Formatting' appears on the screen, release all of the buttons