Resetting the Handset

YOU might need to deregister the hand set first by holding down the red end (hang up key) and keep it down while holding the # key. If deregister wont work use base unavailable and yes, you are sure twice.

List the handset numbers of those working handsets
Using a WORKING handset follow these directions
Resetting Handsets
If you want to register the handset to a different base or
replace a handset with another one, you must first clear the
registration information from the base and the handset.
1. Press and hold [ ] and [#] for
at least 5 seconds, until the
SYSTEM RESET menu
appears.
2. Select the DEREGISTER HS
submenu. WHICH HANDSET?
appears in the display.
3. Move the cursor to select the handset ID to be de-registered
from the list, and then press the OK soft key. DEREGISTER HS
appears. CHOSE A NUMBER THAT IS NOT ONE OF YOUR WORKING HANDSETS
4. Move the cursor to select YES, and then press the OK soft key.
When ARE YOU SURE? appears, select YES. You will hear a
confirmation tone.
PUT THE HANDSET IN THE BASE CHARGER AND IT SHOULD REGISTER

Hope This Helps - It worked for me

First make sure you clear the registration on the problem handset. Go to the handset, menu, select de-register; and select yes. Bring your handset next to your base, on the base unit menu select register handset. On the handset, hold the # for 2 seconds. You should see registration completed. Good luck.