I own a Motorola Cordless phone (Model: L402C). The base unit is working properly. The cordless unit gave me the following message "Warning Check Battery". I replaced the battery and the message continue to be displayed and the cordless unit is no longer working. The phone is less than 12 months old and is under warranty. I also cleaned the charger to make sure some dust is not preventing the charging. Can you please help.

SOURCE: Cordless phones will not connect to base.

I just had the same problem and tried several things with no results. Then, I figured I had tried everything else so I just unplugged the battery pack, set the phone down for 25 minutes and then plugged the pack back in. Phone worked again after that.

SOURCE: Motorola Cordless MD400 series - "Out of Range" message on display

change the batteries in the base unit, base unit takes 4 AAA battries

SOURCE: 5.8 GHZ Motorola Cordless Phone, Base w/1 additional phone

This is not a problem with the battery but rather the phone has lost sync. You can get it back by going to the menu screen then going to register. The phone will walk you through from there. Mine lose sync from time to time and I get the same error. I have these phones both in my shop and home.
Hope this helped you out...

SOURCE: cordless phone does not connect

You may need to reset the Telephone Answering System. Complete the following steps to reset the Telephone Answering System:
Step 1: Disconnect the power to the telephone base.

Step 2: Wait a few minutes and then reconnect the power to the telephone base and charger.

Step 3: Set the day and time.

SOURCE: MA355 Cordless Phone Makes Noise when IN CHARGER?

yes this happens to me also. I haven't tried anything to fix it but it is darn annoying.