I just bought a Uniden Dect 6.0 (4) handset bundle. My problem is a loud humming in the background when the handsets are in use. I thought it might be a 'ground' problem. Radio Shack told me it was interference from the internet line and sold me a filter for $20. It did nothing, I still have the loud humming in the background when we are on calls.

SOURCE: uniden 6.0 with 2 handsets

Unplugged all telephones but one and still had trouble so I called Verizon. They discovered it was an incoming line that was malfunctioning. However, the Uniden telephone system still does not work and since it is still under warranty I will contact them.
To continue the solution of my telephone problem - It was discovered that the incoming line was indis repair. After it was repaired, I again set up my Uniden telephone system. It works with no problem. The word "searching"still appears on the face of the units, but it can be overcome with normal activity on the units.

SOURCE: Uniden 5.8 GHz Digital cordless w 3 handsets interference background noise no matter where we are in the house or which handset we use

This is possibly caused by the internet connection.
Try new ADSL filter.
You need one each per device.

SOURCE: I have a Uniden DECT 6.0 phone set. I damaged one

On the base, open the menu and select Register Handset. The display should say Handset Registering.
On the handset you want to register, press and hold # until the handset display says Handset Registering (about 2 seconds).
1.
2.
DSL filterTelephone cordTelephone wall jack
45
In about 30 seconds, the handset display should say Registration Complete. Press Talk/fl ash and make sure you get a dial tone.
If...
Try...
you don't hear a dial tone
the display says Registration Failed --
Making sure the handset is fully charged, then start over at step 1.

SOURCE: Handset won't connect to base unit

Try placing the handset in the base unit to register it. Then see if it is found by the base.

SOURCE: I bought 2 sets of

Jill,
The only logical way is to get the additional handsets to communicate with the base. And to do that you must set them to the frequency of the base. You Use guide will tell you how to set the frequency n the handsets to talk to the base. They way you set the original ones.
BUT, I doubt if this will work. Why? Because if that was the case if you and I had the same company phones, then I would stand outside your home and set the same frequency on my handset and misuse your phone. A great security risk in today's world. But there is no harm trying it out. Just set the same frequency on the additional ones as the base. If it works, then you will have that many more intercoms too. And do let me know if it works.