To reset the system to factory defaults.... open the front panel, at the lower right, there should be a small battery pack. This keeps the RAM active. Next to the battery is a little red button.
Pull the battery pack out, unplug the AC, make sure the system is not on a UPS, then hold the red button in for at lease 30 seconds - or longer.
This will restore to default mode - all preprogrammed speed dial will be gone, the DSS field will be over written.
The voice mail system has a simular power source to keep the system alive during a power failure.