Want to be clear about the handset and telephone base set because base set is not responding to handset.

Add and register a handset
Each newly purchased handset must be registered to the telephone base before use. When first purchased, each expansion handset shows NOT REGISTERED.
You may need to charge the phone first.
To register a handset:

·Place the unregistered handset in the telephone base or charger. Press Find for 4 seconds. If the headset screen doesn't light up after the 4 seconds. You will need to charge it some more.
·After making sure it is charged set the telephone on the main base again press and hold the Find button for 4 seconds ( until the red light IN USE comes on) Then release the find button. The headset will show PLEASE WAIT; this takes about 10 seconds to complete the registration. The hand set will show HEADSET REGISTERED and the headset number assigned, you will hear a beep when the registration is finished
·When the registration is complete, the newly registered handset is assigned the next available handset registration number. For instance, if you already have HANDSET 1 and HANDSET 2, the next registered handset is HANDSET 3.
·Makes sure no other headsets are in use at the time or the registration may fail.
·You cannot register a handset if any other system handset is in use. Only one handset can be registered at a time.

Hi,
Try the following:
1/ Make sure the power cord is securely plugged in at the base and that it's powered on ie: light is on and working etc..

2/ Reset the telephone base. Unplug the electrical power. Wait for
approximately 15 seconds, then plug it back in. Allow up to one minute for the cordless handset and telephone base to reset.

3/ Place the cordless handset in the telephone base for one minute to allow the cordless handset and base to synchronize.



Manual available here:
at&t el52300 manual


-Charles

If you want to replace a handset or re-assign the designated handset number of a registered handset, you must de-register all the handsets and then individually register each handset.
To de-register all handsets:
Press and hold
FIND HANDSET
on the telephone base for about 10
seconds (until the IN USE light on the telephone base turns on and starts to flash), then release FIND HANDSET
.
Press and release
FIND HANDSET again. (You must press FIND HANDSET
while the IN USE light is still flashing. The IN USE light flashes for
approximately seven seconds.)
All system handsets show
CONNECTING...
The de-registration process
takes about 10 seconds to complete. All handsets show
NOT REGISTERED
when the de-registration completes.
If de-registration fails, you might need to reset the telephone and try again.
To reset:
Pick up the registered handset and then press
/FLASH
.
Press
OFF/CLEAR
and place the handset back in the telephone base.
-OR-
Unplug the power from the telephone base, then plug it back in.
note:
You cannot de-register a handset if any system handset is in use.

After de-registering, proceed with registering process:
If the handset screen does not show LOW BATTERY
or HANDSET X, you will need to charge the handset for at least five minutes before registering to the telephone base.
To register a handset:
Place the unregistered handset in the telephone base or charger. If
PRESS FIND HS 4 SEC ON BASE
does not appear on the handset screen after a few seconds, remove the handset and place it in the telephone base or charger again.
On the telephone base, press and hold
FIND HANDSET
for about four seconds (until the red IN USE light on the telephone base turns on) and then release the button. The handset shows
PLEASE WAIT...
and it takes about 10 seconds to complete the registration. The handset shows
HANDSET X REGISTERED
(X represents the handset number assigned)
and you hear a beep when the registration completes.

Model LS6305: How to register a handset:

1. Place the non-registered handset in the telephone base or charger. If 1.Press FIND HS on base 4sec and 2.Then press # on handset do not appear alternately on the handset screen after a few seconds, remove the handset and place it in the telephone base or charger again.
2. On the telephone base, press and hold FIND HANDSET for about four seconds until the red IN USE light on the telephone base turns on.
3. Remove the handset from the telephone base or charger and then press # (pound key). It displays Registering...Please wait. It takes about 60 seconds to complete the registration. The handset displays Handset X Registered and you hear a beep when the registration process completes.

• If the registration fails, the handset displays Registration failed for a few seconds, then 1.Press FIND HS on base 4sec and 2.Then press # on handset alternately. To reset the handset, remove it from the telephone base or charger and place it back in. Try the registration process again.

Model ip8300: How to register a handset:

1. Press and hold FIND HANDSET for at least 10 seconds on the telephone base. The VMWI and LINE IN USE lights will blink alternately indicating that the telephone base is in registration mode.
2. To display the telephone feature menu, press the Menu softkey from the main idle screen.
3. Press the up, down, left and/or right NavKeys to choose Registration Settings then press the Select softkey or center NavKey/SEL.
4. Press the up or down NavKey to choose Handset Settings then press the Select softkey or center NavKey/SEL.
5. Press the up or down NavKey to choose Register to Base then press the Select softkey or center NavKey/SEL. A list of locations for assigning telephone bases will be displayed.
6. Press the up or down NavKey to choose an empty location and press the Register softkey or center NavKey/SEL.
7. The handset will display Searching while it searches for the telephone base. If the search is successful, a code from the telephone base will be displayed. To register the handset to the telephone base, press the Register softkey.
8. In the Enter PIN box, enter the system PIN. The default PIN is 0000. Press the Apply softkey.
9. Handset registered will be displayed when the registration process is complete. The screen will return to the registration settings menu. Press OFF to return to the idle screen.

Model ip8301: How to register a handset:

• Before registering the handset with the base station, press the FIND HANDSET button in the base till the VMWI and LINE IN USE lights blink alternatively. When the lights start blinking, register the handset. The default PIN for registering the handset with the base station is 0000.
  

1. Press Softkey Menu from Home screen
2. Choose the Settings icon from Menu screen and press center Navigation key/SEL to display the settings menu options.
3. Press the up or down Navigation Key to choose the Registration Settings menu option and press the center Navigation key/SEL to select.
4. Press the up or down Navigation Key to choose the Handset Settings and press the softkey Select to select.
5. Press the up or down Navigation Key to choose the Register to Base and press the softkey Select to display a list of available base stations. Choose a base station from the list and press the softkey Select to select.
6. The handset will search for the Base within its range. During this time, the searching screen will be displayed.
7. If the search is successful, the Base station number of the base will be displayed.
8. Press softkey Register. Enter the PIN and press softkey Apply to save and subscribe to the base station.
9. To cancel the entire operation, press OFF key anytime.
   

I’m happy to help further over the phone at https://www.6ya.com/expert/jeremy_69f3cc28d95bf514

Please try the following procedure to register the handset:

1. Place the unregistered handset in the telephone base. If PRESS FIND HS 4 SEC ON BASE does not appear on the handset screen after a few seconds, remove the handset and place it in the telephone base again.
   
2. On the telephone base, press and hold FIND HANDSET for about four seconds (until the red IN USE light on the telephone base turns on) and then release the button. The handset shows PLEASE WAIT... and it takes up to 60 seconds to complete the registration. The handset shows HANDSET X REGISTERED (X represents the handset number assigned) and you hear a beep when the registration completes.

When the registration is complete, the newly registered handset is assigned the next available handset registration number. For instance, if you already have HANDSET 1 and HANDSET 2, the next registered handset will be HANDSET 3

Getting startedReplace a handsetIf you want to replace a handset or reassign the designated handset numberof a registered handset, you must deregister all the handsets and thenindividually register each handset.To make deregistration easier, read all of the instructions on this page beforeyou begin. To deregister all handsets:Press and hold FIND HANDSET on the telephone base for about 101.seconds (until the IN USE light on the telephone base turns on and starts toflash), then release FIND HANDSET.Press and release FIND HANDSET again. You must press FIND HANDSET2.while the IN USE light is still flashing. The IN USE light flashes forapproximately seven seconds.All system handsets show CONNECTING... The deregistration process3.takes about 10 seconds to complete. All handsets show NOT REGISTEREDwhen the deregistration completes.If deregistration fails, you might need to reset the telephone and try again.To reset:1.Pick up the registered handset and then press talk/flash.Press OFF/CLEAR and place the handset back in the telephone base.2.-OR-1.Unplug the power from the telephone base, then plug it back in.You cannot deregister a handset if any system handset is in use.

Press the END button to exit or erase the message. Press Menu/Select. Scroll uo or down to find Register Base. Select the option, select it and put the handset in the charging cradle of the base. A confirmation beep will sound.

Does the Phone Power Up?
1. When you pick up your GE cordless phone and press a button, the phone should beep as a confirmation that a button has been pressed. If the telephone handset does not beep, or if it does not respond to any buttons, the battery may be in need of a recharge. Verify that the cordless phone base unit is plugged in to an electrical source, then place the handset into the charging cradle. If no lights illuminate on the phone, or if charging fails to correct the issue, clean the metal charging contacts on the cordless handset and place the phone back on the charger. If it still fails to respond or to charge, the unit may need to be serviced or replaced.
Does the Battery Hold a Charge?
2. If the cordless phone charges up and appears to be ready for use but dies after only short periods of use, the internal rechargeable batteries may need to be replaced. Replacement batteries are widely available at electronics and discount retail stores. Note your phone's exact model number, as this number will be necessary when purchasing a replacement battery.
Listen for a Dial Tone
3. If the GE cordless phone powers on, and beeps when you press keys, but no dial tone is heard, one of three problems is the likely culprit. The phone's batteries may have an insufficient charge to fully power the phone, a condition that can be corrected by simply placing the phone back in the charger. If the batteries are charged, but no dial tone is heard, it is also possible that another telephone in your home or office is "off hook" or in use; verify that all telephones, fax machines and modems are securely on the hook. If there is still no dial tone, your telephone service may have been disconnected or require service by your local telephone company.
Listen for Static
4. On some GE cordless phone models, particularly older models, static may be heard if the radio signal between the handset and base unit experiences interference. Common sources of interference include unshielded electronics, appliances, and radio transmitters, as well as high-tension power lines. If your phone offers an option to change channels, try several different channels to see if the static is eliminated. If static continues to exist, or if static can be heard on wired phones sharing the same line, your telephone line may require service.
Verify Your Handset is Registered
5. Some models of GE cordless phones may display a message on the display screen that the handset needs to be registered. If this message appears, move the handset close to the base unit and press the "Prog/Mute" button near the bottom of the handset. Next, press the "Pager" button on the base unit (your handset may instruct you to perform this step). Finally, press the "Prog/Mute" button on the handset one more time to complete the registration process. Your GE cordless phone should now work correctly.The GE Cordless Phone brand is designed to deliver the conveniences of cordless home service with quality sound and features. There are many models of the GE Cordless Phone. The most current models, including the DECT 6.0, have standard features such as call waiting, caller ID, visual ring indicator, phone number memory and digital answering systems .
   
6. Before you use your GE Cordless Phone, ensure that you have all the parts. Your cordless phone package should include the phone base, a handset battery pack, a battery compartment cover, the cordless handset, a wall mount bracket, and a telephone line cord and AC power adaptor. Once you have confirmed that you have all of your parts, insert the handset battery pack into the handset and replace the battery compartment cover. Insert your telephone line cord and the AC power adapter into the phone base. The phone base identifies the telephone line as TEL LINE and the adapter as AC. Plug the phone base into the electrical socket. Place the cordless handset into the base. Allow the phone to charge for at least 16 hours before its first use. Failure to fully charge the battery before its first use will result in poor battery performance.
7. Once you have fully charged your phone, you can proceed with setting up your phone's features. If your cordless phone includes an answering system, complete this setup process first. Turn on the answering system by pressing the on/off button. The message indicator light will display. The cordless phones are defaulted to English as the selected language. To change your language preference, press the "Skip" button for two seconds. The language will change to Spanish. Next, you will set up your voice time and day stamp. While the answering system is on, press the "Answer" button on your handset for two seconds. The phone will enter the answerer mode. Press the "8" key. This will take you to the time/day setup menu. Use the number keys to set the hour and minutes. Use the volume up and down arrows to set the AM/PM. Press "#" to save. The cordless handset will also require that you give it a name. Press the "Mute" button for two seconds until the handset reads "ENTER NAME." Enter the name of the handset by using the touchtone keypad. Press the "Mute" button to save. Enter the default area code by pressing "Mute" until the phone displays "AREA CODE." Enter your three-digit area code and press the "Mute" button to save.
8. GE Cordless Phones are designed to be simple and convenient to use. To place or answer a call, press the TALK button. You can pre-dial a number by dialing the number before pressing the TALK button. When you are ready to make the call, press TALK and the phone will dial automatically. To answer a call that is waiting on the other line, press the FLASH button. If you experience call interference or bad reception while on a call, press the CHANNEL button until your line clears
9. 
   

Dear....Fixya Team,

I'm Soontaree. Thank you for your reply email. I would like to let you know i'm already solved this problem. I just have a look from the Uniden Manual so i just follow them detail like as below,

Thank you for your help i will contact to you later if i have anything problem.

Have a great day.
Soontaree



Registering DSS7905 Accessory Handsets
Your phone supports a total of ten (10) cordless handsets. Handset(s) supplied in the box with the phone base are
pre-registered at the factory. If you purchase an accessory handset, you need to register the handset before use.
Only one handset can be registered at a time. When charged, pre-registered handsets display a handset ID number.
Handsets that have not been registered display.

Before registering the DSS7905 accessory handset, the battery pack MUST be charged for 15-20 hours.
To register the DSS7905 handset:

Place the handset in the telephone base charging cradle to begin registration.
During the registration process, HANDSET REGISTERING will appear in the LCD. When REGISTRATION
COMPLETE. is displayed; the handset has been registered to the base. If REGISTRATION FAILED. appears,
please try these steps again. When you register an accessory handset to the base, the handset ID will be
assigned.

Note:
If an accessory handset has ever been registered to a base, you must reset the handset before you can
register it to a new base. See Resetting the Handsets on page 24.

To deregister ALL handsets:
Step 1: Press and hold FIND HANDSET on the main telephone base for 10 seconds.

Step 2: When the IN USE light starts to flash, release the FIND HANDSET button.

Step 3: Press and release FIND HANDSET again. The IN USE light will turn off and in a few seconds, all handsets will be de-registered.

Step 4: All handsets will show NOT REGISTERED.

To register a handset (register 1 handset at a time):
Step 1: Place the handset in the base unit or a charger that is connected to a power outlet. If PRESS FIND HNDST 4 SEC ON BASE does not appear on the screen after a few seconds, lift up handset and place in base or charger again.

Step 2: On the telephone base, press and hold FIND HANDSET for four seconds. Release FIND HANDSET once the IN USE light comes on.

After a while the handset will emit a series of tones, the newly registered handset will be assigned the lowest extension number not previously assigned to another system handset (1-4) when successfully registered. Repeat Steps 1 and 2 if the handset does not successfully register to the telephone base within two minutes.

REPEAT THE REGISTRATION PROCEDURE FOR EACH SYSTEM HANDSET.

Make sure the handsets are not receiving interference from other appliances, cell phones, etc.

IF deregistering does not work .. you will need to reset the entire system
Step 1: Disconnect the power to the telephone base.

Step 2: Disconnect the handset battery.

Step 3: Wait a few minutes and then reconnect the power to the telephone base.

Step 4: Reconnect the handset battery

Step 5: Wait about one minute for the handset to re-establish a link with the telephone base and then test the phone again.
You will need to set the time and date again after resetting the entire system.