We had the same problem and luckily found this page:
http://www.fixya.com/support/p129847-ge_21009ge3_21028ge3_21028ge3/manual-13404
When your handset displays:
HANDSET NEEDS REGISTRATION
1. Press the MUTE/PROG button and HOLD BASE PAGE WAIT FOR BEEP shows in the display.
2. Press and hold the page button on the base of Models 21018, 21028 or the PAGE/INT button on the base of Model 21098 until you hear a long beep. The handset displays HANDSET # REGISTERED followed by HANDSET NAME. HOLD BASE PAGE WAIT FOR BEEP HANDSET NEEDS REGISTRATION HANDSET # REGISTERED HANDSET NAME...
mikelinear
January 2007