First make to get a compatible handset.
For the Uniden D1660-2, the compatible handsets are DCX160 and DWX337. Make sure that you have not exceeded the number of handsets that the phone can support. The D1660-2 can support 12 handsets unless you use any DWX337 handsets. Then the limit is 6. (If you have more handsets, you will need to deregister some of them first.) Then allow the handset to charge. If the handset is reading "not registered", then place it into the base unit. You should see "Handset Registering" for about 30 seconds; then it will read Registration Complete. The manual for the Uniden D1660-2 is available here: http://www.uniden.com/content/ebiz/uniden/resources/ownersmanuals/D1660-2om.pdf
. This is the most common Registration for Uniden phones; there are a few exceptions (depending on the base or the replacement handset). Check the appropriate manual (see below to find the manuals).
For the DECT4096, press Menu on the base and then navigate to Handset Register. When you see Handset Registering on the base display, press and hold # on the handset to be registered until you see Handset Registering. (It takes about 2 seconds.) After the display changes to Handset Registered, check for dial tone.
On a few Uniden phones, there is a Page/Handst Register button on the base. On those you navigate through the Handset menu to Handset Register (Menu > Handset menu > Handset Registration). Then press and hold the Page button until you hear a beep. Then wait for the registration to be completed.
Most Uniden manuals are available here: http://www.uniden.com/scat/cordlessphones_mnl
. Select your type of Uniden phone and scroll through the list to your model or use the search box to directly search for your phone and the manual.
I hope this helps.