AT&T SYSTEM 25 Manual page 114

This feature applies to multiline voice terminals only.
The lights and ringer on a voice terminal can be tested if the voice terminal has a
Test/Program Switch.
If so, the switch must be held in the "T" position to test the lights and ringer; the
switch will automatically return to the midpoint when released.
For display voice terminals, when the switch is moved to the "T" position, the display
shows 16 blackened squares. After the switch is returned to midpoint, the display unit's
alarm clock sounds (three short beeps).
Testing Your Terminal
1
Slide the switch on the left side of the voice terminal
to "T".
NOTE:
the "T" position.
The red and green lights alternately light, and the ringer sounds.
2
If your switch has a spring, release the switch. If there
is no spring, slide the switch back to the midpoint
between "P" and "T".
Testing a Voice Terminal
some voice terminals, the Test/Program switch has a spring.
On
If your switch has a spring, hold the switch in
Testing a Voice Terminal 2-101