I didn't see a response from my earlier note, but wanted to send the following before I take an afternoon break.
From the sound of those symptom, and assuming the system is more than about 1-2 years old, I would think that the battery pack in the handset(s) needs to be replaced (getting worn out).
If that is the case, you should be able to find replacement battery pack(s) at the same shop where this system was purchased.
OR - I did a search of Amazon and found quite a few replacement batteries listed. I have the results of that search below.
AT&T CRL32102 Replacement Batteries
Results Here --> http://amzn.to/1HQAbLJ
Please let me know if you have any more details for me or any other questions.
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Thanks, ***** *****