After some googling, trial and error and an email or two to Uniden I have found my answer.
The handset/base keeps a record of who it’s registered to. The base also keeps a record of how many handsets are registered to it.
Using the de-register function on the handset deletes the handset from the base (decrements the count) and deletes the base info from the handset. All this requires that the base and handset can communicate with each other.
To remove the base data from a handset when out of range of the base, press and hold the ‘end’ button and the ‘#’ button simultaneously until the phone beeps. It will now be de-registered from the base.
However, the base will still have this handset in it’s record.
To de-register all handsets from the base, remove power and the base handset. Press and hold the ‘flash/find hs’ button while applying power. Wait for the beep and release the button. The charge light will now flash. Sit any handset in the cradle. When the flashing stops, all handsets are inhibited from connecting.
However, the handset still appears to be connected to the base unit. Pressing talk will give an ‘Unable to connect’ message. To re-register the handsets to the base, you have to de-register the handset first with the two button method.
Of interest, pressing ‘*’ and ‘#’ on the handset while reconnecting the battery is suppose to reset the handset. Doing the same on the base unit puts you in some sort of diagnostic mode. Press speaker to exit.
Link: Whirlpool Forum
Uniden: DSS 7855