AT&T System 25 - Manual (Page 66)

Testing Your Telephone
You can test the lights and ringer on your telephone if you have
a Test/Program Switch. On some telephones, the Test/Program
switch has a spring. If so, you must hold the switch in the “T”
position to test the lights and ringer; the switch will
automatically return to the midpoint when released.
Testing
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Slide the switch on the left side of your telephone to “T”.
The red and green lights alternately flash, and the ringer sounds.
NOTE: If your switch has a spring, hold the switch in the
“T” position.
If your switch has a spring, release the switch. If there is no
spring, slide the switch back to the midpoint between “T”
and “P”.
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