Reboot the Phone

Press and hold the dial pad keys 0, 1, and 3 simultaneously for about three seconds, or until you hear a confirmation tone.

Restore Factory Defaults

When the handset is booing and displays the 'starting application' window press 'cancel'.

Press and hold the dial pad keys 1, 3, and 5 simultaneously until the password prompt appears. 

If the admin password is known, enter it now to factory default the handset. 

If the admin password is unknown, then enter the MAC address of the handset in order to perform a factory default. 

Resetting to factory defaults will also reset the administrator password to 456.

The following article provides full details on the above across the range of handsets as not all models support the above shortcodes.