To reset (restore factory settings) your phone try this:
Press Menu > Setting Up Your Phone > Setting the System > Security > Restore Defaults

Note: You should input the phone lock code to enter the Security menu. The default lock code for an inactivated phone is 0000. Once you have activated your phone, the default lock code will be the last 4 digits of your phone number. If the updated lock code is lost or forgotten, please contact your service provider.