AT&T System 25 - Manual (Page 51)

Personal Message Waiting
You and another person may have been assigned a button that is
used to signal each other. If so, a waiting message is indicated
by the light next to
being lit. This light can be
turned off at either telephone.
Sending a Personal Signal
1 Do not lift the handset.
2 If the light next to the button is off, press it.
Canceling a Personal Signal
1 Do not lift the handset.
2 If the light next to the button is on, press it.
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