Which Uniden phone do you have? Some should register with the handset in the base unit cradle for 30 seconds to a minute. Others need to be within a few feet of the base but not in the base charging.
For the models that need to be registered with a multiple step process, first make sure that the handset is fully charged; this could take 15 hours. (Make sure the battery is properly connected; if it is loose, the registration may fail. If the battery may have been sitting in a warehouse for months, you may want fresh batteries even for a new phone.) Make sure that the base is plugged in (power and telephone line). I like to check the phone line with a basic corded phone for dial tone if there are any issues with a cordless phone. With some Uniden phones, a registration problem can be cleared up by disconnecting the battery and then reconnecting it before attempting to register the handset. (Make sure that you do not have more registered handsets than the base can deal with. You may need to deregister all of the handsets and start over. Make sure that the handset is compatible with the base.)
The rest of the process requires you to have the handset close to the base and be relatively quick. (You need to tell the handset to get the registration information from the base and the base to send the information before they time out). For the D1688, at the base, you need to press Menu and go to the Register Accy submenu. Press Select. Then press and hold # on the handset you need to register until you see Registering Handset. Wait for the Registration Complete (or Failed) message.
For the DECT1060, the procedure starts at the handset. Press Menu and navigate to Add. Settings. Press Menu/OK and navigate to Register and press Menu/OK. Select the base number and press Menu/OK. Then press and hold the Find HS button on the base for 3+ seconds until the unit enters registering mode. Now on the handset, you need to enter a PIN that matches the one set for the base. (The default is 0000.)
If you still have problems with registering the handset, then it's likely that the transmitter or the receiver for either the base or the handset may have a problem sending and receiving the codes that synch the base and the handset. Most Uniden phones are not meant to be user serviced for this type of proble. You will need to contact Uniden if the phone is under warranty or decide if you want to replace or repair the phone through Uniden's support.
The manuals for all Uniden DECT 6.0 phones are available as PDFs from Uniden. See http://www.uniden.com/bin/venda?ex=co_disp-view&bsref=uniden&scat=dectphones_mnl
for a list of all models or search for your specific model. On the manuals pages, the models are listed in numerical and alphabetical order.
I hope this helps.