One of the handsets work and the other kept saying register. Despite trying to register, putting in the pin 0000, and pressing the page button on the charging unit, it still kept saying register. So after reading Jock 157's answer, I tried it - unplug mains charger, plug it back with page button depressed, then do a registration on the faulty one again and then suddenly it was OK.
Joan Guyll, March 2014