These phones are end of life because of poor product quality, the best thing to do is buy a new equivalent (8242 or 8262).
But if your really want to reset the device to factory default, you can do this by the menu or by the procedure below:
1) Press the OK button the access the menu
2) When the menu icons are displayed form the following code (this code will not be displayed on your screen): *632* (or *7378423*)
3) Select EEprom reset
Good luck,
Grts
If I'm ever on life support, unplug me. Then plug me back in, see if that works...