AT&T ML17959 - Manual (Page 9)

Getting started
4
Optional backup battery installation
Install three AA alkaline batteries (purchased separately) to use this telephone
in the event of a power failure. If power fails with working batteries installed,
all functions work properly except for the display illumination.
If power fails and no batteries are installed, you can make or answer calls on
line 1 using only the handset. Last number redial and speed dialing are also
supported, but the screen display will be dimmer than usual.
Install the batteries as shown below:
Press the tab and pull to remove the battery compartment cover. Insert
three AA alkaline batteries (purchased separately) following the polarity
markings in the battery compartment. 1.
Replace the battery compartment cover. 2.
NOTE: If no backup batteries are installed and the power adapter is connected, appears
steady on the screen.