In order to do a factory reset simply open the phone and dial “*2767*3855#”

The phone will restart and then do a master reset (make sure the battery is not too low now, that will be bad).

Another way should be “Settings > Basic Settings > Memory Settings > Clear memory > Enter + 1234 and confirm”

Another way is to hold all of the following buttons at once “green dial + power + volume up+ keylock” that will give you a question if you are sure (green dial confirms).