In Use LED flashing constantly

Have you tried removing the batteries and letting the internal power decay for an hour or so - in other words try to get them to forget what they were. Then replace batteries and try again. I guess you've consulted the user manual, is there a reset button hole anywhere on the base unit? The sort you stick a pin into.

You mean unplugged it from the power outlet or the RG45 wall jack? OK so if the battery is fine and you unplugged the power from the source, then the issue is with either the cord itself or the charging unit. Just because the little red light comes on does not mean it is working properly. Put a multi-meter on the charging tabs to double check for continuity. Sometimes and as weird as this sounds, it just takes unplugging a nearly spent power pack/inverter to finish the job so to speak. If you unplugged it from the RG45 then you need to look at the pins in the RG45 wall jack to see if you inadvertently bent one out of shape.Phone won't work if there's no signal. Wish I could help more. Good luck with that issue.

here are some tips from my manual for out of range problems
hope they help
• Only one handset can be used to make or answer a call anytime.
If another handset attempt to do so, the screen will display OTHER
HANDSET ON and then OR OUT OF RANGE. You might press PHONE
or SPEAKER again until no other handset is using the line. Use the call
transfer feature to transfer a call to another system handset
or use the speakerphone feature so multiple members of your
house can be on an active call at the same time.
• Move the cordless handset closer to the base. You might have moved
out of range.
• Ensure that the base is powered up.
• Place the cordless handset in base cradle for 1 minute to allow the
cordless handset and base to resynchronize channels.
• If the cordless handset is in its base and the charging light does not
come on, refer to the charge light is off in troubleshooting guide.
• Reset the base. Unplug the unit’s electrical power. Wait for 15 seconds
then plug it back in. Allow up to one minute for the cordless handset
and base to reset.
• Other electronic products can cause interference with your cordless
phone. Try installing your phone as far away from these types of elec
tronic devices as possible: wireless routers, radios, radio towers, page
towers, cell phones, intercoms, room monitors, televisions, personal
computers, kitchen appliances and cordless phones.

The compatibility setting, in almost 100% of the cases, should be set to "1".  You can find it on the side of the base, it's a little turn wheel that will turn if you stick your thumbnail in it and twist.  I have seen one instance on the Cisco IP phones that requires the compatibility setting to be on 5, so try that as well.
If you have a lifter installed, make sure that the short telephone cable that comes out of the back of the headset's base unit is plugged into the handset port of the telephone (where the handset coily cord is normally plugged into).  You would then plug the handset coily cord into the front of the headset base unit.  Make sure you have that configuration, and try again.
Finally you can try a re-set.  To do this you should lift the headset off of the charging base, make sure the green light on the top of the base is off (the headset is off), and then do the following: 1. Hold down the mute button and the call answer button at the same time. (The mute button is the black little wheel on the top of the headset itself.  If you use the tip of your finger to push that straight in, it will mute the headset) 2.  Keep holding both buttons down until the green light below your thumb on the headset flashes for 2 consecutive series. 3.  Let go.  Wait about 20 seconds then try answering.
If this doesn't work, try a re-subscription: 1. Place the headset back on the charging base. 2. The two microphone volume adjust buttons on the back of the BASE unit should be pressed together.  This will start the red light and the yellow light blinking on the front of the base unit. 3. Hold the mute button down while the headset is still on the charging base.  Hold it down until the red light stops blinking. 4. Disconnect the power to the base so that the red light and yellow light turns off. 5. Plug the power back in after about 5 seconds of it being disconnected and make sure the red light on the front of the charging base is now solid and the yellow light is flashing. 6.  Pick up the headset and try again.

I have two older (2002) model cordless Uniden phones that require the handsets to return to their original factory bases. If they have rested on another Uniden base, even of the same model, there is no dialup tone. Replace to the original base and you should be back in service.

IF it was a used handset it was likely registered to another base. I just switched bases (1line to a 2 line base). I had to "deregister" the handsets from the old base. I am not sure if you can deregister if the old base is not present? With the old base plugged into power I hit the "function key" at the bottom left of the handset and scrolled down to initial settings. Under registration is "deregistration". I selected that and it put the handset back to its default out of the box. Then simply hit the mute button on the New base and OK on the handset. Worked for all 5 of my old handsets.

Hope this helps

I had the same problem. Once I read the owners manual I found the charge light blinks to indicate that you have voicemail waiting to be retrieved via telecom provider. Much to my surprise I have a voicemail account I never requested.

Another mystery solved - I now know why I missed so many messages.

check your handset.. reheat or resolder the components at pcb.

the base of your handset needs to be repair take it to a tech

You are pretty close to the solution. The problem is the charge detection in the handset. All handsets on a cordless phone detect the battery level with an hexadecimal value in the memory. Each hex value equal a volt value of the battery. EX: 3.5v = 9B
3.6v = 9C 3.7v =9D etc... The problem is the hex value in your defective handset are all mess up. So 9B is not 3.5v anymore. Then your handset could last 1hr after foul charge or doesn't alert you before the phone goes dead due to low BAtt.