You can restore the default factory settings for your device; from the standby screen, press Menu Menu key > Settings Settings Icon > Phone Settings > Security > Reset Phone. Scroll down to select Yes, press the Select key key, and enter your Lock Code. Your device will power-cycle and reset back to factory defaults. When your device is done power-cycling, press *228 then Select key from the standby screen to reprogram your device.
NOTE! Using the Reset Phone feature will not delete your personal information; it will restore only settings like default ringtone, keypad tones, and display settings.