AT&T SYSTEM 25 Manual page 104

Signaling (Manual Signaling)
This feature applies to multiline voice terminals only.
Multiline voice terminal users can signal another predesignated multiline voice terminal by
pressing an associated Manual Signaling (SIGNAL) button. A single tone burst is provided
at the signaling terminal. The signaling voice terminal also receives the tone and can use this
feature while at any time, whether on-hook or off-hook. No LED indication is associated
with the Manual Signaling feature.
Signaling the Other Station
Press
[SIGNAL].
Signaling (Manual Signaling)
2-91