AT&T System 25 - Manual (Page 8)

Buttons
Most multiline telephones have
Each is represented in a special
TYPE
dial pad buttons
three standard types of buttons.
way in this guide:
SHOWN AS:
button next to permanent
label with dark background
button next to changeable
label with light background
NOTE: A fourth type of button can be found on the display unit,
and is represented in this way:
Each multiline telephone has System Access buttons that are
used to place and receive calls and to access special system
features. On most phones, these buttons are located on the
upper left portion of the faceplate, and are labeled with your
extension number.
Line Selection
Your telephone should automatically select a line when you lift
the handset to place a call, and automatically select a ringing
line when you lift the handset to answer a call. These options
are set by the System Administrator, and the procedures in this
guide assume that you have both of them. (If you do not have
these options, you must press the appropriate line button at the
beginning of most precedures.)