I’ve had the same problem with all my three handsets in my vacation home.
I believe it’s a design flaw because this only happens with Model KX-TGL433B I have in my vacation home. In my main home I have five handsets of a different Panasonic model, KX-THD564 that have never had this problem.
My high-tech “solution” to the beeping problem is to put a coin behind the handset when it’s sitting in the cradle in order to tighten the loose connection there is when the phone is in the cradle. It works most of the time, haha.