AT&T TR1909 - Manual (Page 5)

Battery installation
Before using the telephone, you should install four good AA
alkaline batteries (not included) to provide memory backup
and enable the use of some features. The four AA batteries are
required for caller ID features and the
IN USE light to work. If
good batteries are not installed, the display only works when you
lift the handset.
If you unplug the telephone from the wall jack, or you lose
telephone service for over 0 minutes, and good batteries are not
installed, the following information is lost: call list, numbers stored
in one-touch and two-touch memory, redial list, and all settings in
the feature menu. It is recommended that you install new batteries
soon after the low battery icon appears.
Follow the steps below to install the batteries:
Press on the tab and lift to open the cover of the battery
compartment. .
Lift this
tab to
open.
Press this
tab to
release.