NY presented the following explanation :
The BT 8500 set that we have replaces some Gigaset phones which had
horrendous problems with failing to answer when you picked the phone off its
rest and pressed the answer button if it failed to auto-answer, and with
batteries dying with no warning.

I have a set of those and no problems like that - though like yours the
original cells were unpredictable, so I soon replaced them with new.

We sometimes find it fails to connect, to charge, when dropped in the
cradle - a nudge sorts that.