You might try to de-register the handset and re-register
it to the base. Be sure the handset's battery is well
charged, a weak battery can impede the registration.
( I ) To de-register the errant handset:
1. Press and hold the [END] and [ # ] keys
at the same time until the screen displays
"System Reset, Deregister Handset"
2. press [select]
3. scroll to "Base Unavailable"
4. press [select]
5. display says "Not Registered"
( II ) To register the handset to the base:
1. open the base display's [menu]
2. scroll to "Register Accy."
3. press [select]
4. on the handset, press and hold [ # ] until
the handset display says "Registering"
5. In ~30 seconds, handset display will say
"Registration Complete" or
This is described on page 22 of the manual.
A free copy of the User's Manual can be obtained