i have the same problem and it has happened infrequently several times. you probably have to deregister the handset and re-register it and rest the base and handheld to communicate since my model is different from yours there may be differences in how to go about it but for my model the suggestion is as follows:
go to www.uniden.com
for customer support here is a direct link to some frequentlt asked questions http://www.mycusthelp.com/uniden/supportfaq.asp?sSessionID=&lstFilter_a=&lstFilter_b=&lstFilter_c=&txtCriteria=hanset+won%27t+connect+to+base&optcond=2&iStartCount=1
DCT Series Multi Handset Phone - reset to have base and handsets communicate
I have a DCT Series Multi Handset phone and one or more of the handsets is not working, what should I do?
Try resetting the phone system by using the following steps:
1) Enter the "System Reset" menu, then "Deregister HS" submenu.
2) Press volume/up or down arrow keys to select the handset ID to be de-registered from the list, then press the OK soft key.
3) Press the OK soft key. You will hear a confirmation tone. When de-registration is complete, "Deregistration Complete" appears.
4) After de-registering the handset, move to "Replacing the Base Setting" below. When you go through the deregister procedure if you get "out of range", That is okay. Go ahead and go to the next step.
Replacing the Base Setting
1) Enter the "System Reset" menu, then "Replacing Base" submenu. "Do you want to replace Base?" appears.
2) Press the OK soft key. You will hear a conformation tone. The base information will be deleted. When replacing the base is complete, "Please Register" appears, register the handset to the new base.
If your handset is reading out of range. Check the the main base and make sure you unit has power. This can be done by placing the handset in the main base to see if the charge in use light comes on.
If your main base isn't getting power please make sure the ac adapter is plugged in and is warm to the touch. If not the ac adapter would be damaged.
To resolve out of range follow the below steps.
Press and hold the end key.
Press and hold the # key.
Release the keys and select replacing base and press select.
You will see Replace base?
Select Yes and press select.
Now place the handset on the base and it should register to the base and allow for calls to be made.