I'm told that you can perform 2 kinds of reset:
Unplug the base station and also the batteries from the phone for at least 30 seconds
or the factory reset:
1. Power off the base station.
2. Press the 'page' button and power on whilst pressing the
'page' button. Keep holding the button after power on until base starts bleeping
3. Release the 'page button when the base unit stops bleeping.
4. Press the page button five times in quick succession.
5. That's it ! Full factory reset completed.
(Thanks to BT and Google for the answer)
Mario Miniaci, August 2005