Have you pressed and held the paging button on the master base unit for 5 seconds before attempting to register a handset? This is necessary for the base unit to send out it's I.D. signal.
If doing this doesn't resolve the problem I suggest you do a reset by following these instructions...
Press "menu" and scroll down (centre key three presses) to "advance set" then press "menu" again and scroll down (center key eight presses) to "reset unit" then press "menu" again and then "menu" to confirm. The reset doesn't remove your phonebook or call log but does reset all other things.
You can then try to register each handset as follows (if they don't sort themselves out)........
Press and hold the paging key on the master base unit for five seconds then, within one minute, take the first handset and press "menu" and scroll down to "advance set" then press "menu" again scroll down to "registration" then press "menu" again and enter the pin number (default 0000)and press "menu" to confirm..
If all is well the phone will register and you can repeat the same for each handset (don't forget to press and hold the paging button for 5 seconds before starting each handset).
Hope it works.
Tony, August 2007