It appears that the one handset has become orphaned and
must be de-registered and then re-registered.

1. To de-register the orphaned handset from the old base:
When the orphaned handset says "Connecting..." or "Out of..."
enter *331734 [menu]
and it will soon beep and display "Not registered"

2. To register the new handset to your base:
press the "Handset Locator" button on the base and hold
it until the "in use" light comes on ( ~ 4 seconds ),
then release the button and set the new handset into
the base phone cradle. After ~ 30 seconds it will beep
and display "Handset #" (next in sequence).

3: Note: the base handles a maximum of 12 handsets.

If your system was fully populated ( ie, a handset is
already # 12 ) you will have to clear all the handsets
from the base and re-register them all in sequence.
( it is the only way to 'rescue' the places occupied
by the lost handsets ).

To clear the base of all registrations:

1. Press and hold the the "Handset Locator" button
until the "in use" light lights and then starts to blink
( ~ 10 seconds ), release the button --
then press it again briefly while the "in use" light
is still blinking.

2. The base will beep and all handsets that were registered
will display "Not Registered"

3. Follow the instructions above for each handset
in turn to re-register them:
press "Handset Locator" ~ 4sec ( in-use light on ),
set handset into the base cradle, etc.

The manual is available from: www.manualslib.com
but does not fully describe these procedures.

Good luck !