The following has happened several times (My batteries are new and always fully charged. (I replaced all of the original NiCad batteries hoping that would solve the blackout problem, it did not).
My wife and I are on a call using 2 handsets. One of the handsets suddenly goes completely black and unresponsive. I place it in the charger and it returns to life within 10 minutes. For example, my wife and I were on an hour long call during which 2 of her handsets went dead. We have 4 handsets and she had to change handsets twice. The handset I was on stayed alive during the call. This can also happen when only one handset is in use, and then I loose the connection.
I assume that the handset reboots once placed in the charger cradle. What is wrong with the handset software???