Had same problem, got the following info from BT - worked a treat!
Base reboot procedure
1.Turn off power to base
2. Hold down PAGE button on base
3. Without releasing PAGE button, turn power on
4. You will hear a series of beeps.
5. Wait till the beeps finish, release page button and then press 5 times very quickly.
You should then hear a confirmation tone. If you do not, you will need to start the process again.
Handset reboot;
1. Slide off the battery cover
2. Disconnect battery from handset by giving a tug on the wires and the white plug should pop out from inside of the handset.
3. Hold down the options button and the INT button together
4. Without releasing the options and INT buttons re-connect the battery
5. You should hear the handset beep
6. Slide battery cover back on
Re-registration of handset to base station
1. Press options button.
2. Press del button ( on the left hand side of options, handset should display registration)
3. Press options twice ( handset should display searching 1)
4. After short delay handset should display base pin
5. Enter 0000 and then press options, after short delay handset should display standby screen.
6. The handset should then be reregistered to base.
7. For any additional handsets, press the PAGE button down for approx. 10 seconds, you should hear a melody. Then repeat instructions 1 to 5.
Paul M, December 2004