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

Nokia 6600 hard reset



Hard reset Soft reset

Hard reset Soft reset

Nokia 6600 hard reset

The method of performing a Nokia 6600 hard-format using key combo:

   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 

Nokia 6600 hard reset