I am having a problem with my AT&T cordless phone, 5.8 GHz, Model E5813B. I can't get the phone to connect to the base. I keep getting the message on the screen that the phone is connecting and other handset is on or out of range. They are not on or out of the range. The other handsets are not connected because they don't even have batteries in them.

SOURCE: "searching" message even when handset on the base

Found the following in AT&Ts FAQs for your phone, I'll leave only what might be relevant: # Charge the handset for 12 hours. After the first charge, the battery will quick charge in only 8 hours. Re-cradle the handset nightly for recharging. The battery will not overcharge. # Connect telephone line from base to unit to modular telephone jack. If cord is already plugged in, unplug and reconnect at both ends. Also, you may want to try another cord from a working telephone. See the owner's manual under "Connecting to Phone Line." (you can get the manual here on the product page in case you lost it) # Try a working phone at the jack. If another phone has the same problem in that jack, the phone jack is defective. Good luck, hope you work it out

SOURCE: 5.8 ghz mulit-handset cordless phone

How do you get the ringer on this phone to work?

SOURCE: AT&T Model # 1487 Phone

http://telephones.att.com/telephones_ui/support/support_index.cfm

Go to the above web site. It helped me.

SOURCE: out of range shows on main handset of cordless phone

Unplug the battery. Hold down The * (star) and the #(hash) keys simultaneously. Re-connect the battery, phone will beep and screen will be blank. Press the voicemail (righthand silver key under the lcd screen). As above EEPROM/RAM message will appear. Place handset on base and your problem is solved.

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.