Restoring communication with a display type handset
If the handset for what ever reason has lost communication with the Fire Control valve, when the
handset is held like a firm handshake the green light will be permanently lit but will pulse brighter.
In addition the communication symbol 
and other symbols will be missing
from the display and it will appear as shown.
To reconnect do the following :-
Reset the handset to clear the old pairing and enable it to accept a new pairing:-
Press and hold the set button and keep held for around 3 seconds
until you hear a second beep. Release the set button and PROG
appears at the top left of the handset. SETUP should also be flashing
at the top right corner of the display.
When SETUP is flashing, press and release the SET button to enter
the menu. H24 (or 12) , will be flashing. Press and release the SET
button about 9 more times until the display shows CA with a flashing 0.
Press and release the + or – button once to change the CA0 to a CA1.
Then press and release the SET button once again. The display will
now show 7ESC r4 (or some other number next to the r).