AT&T MERLIN LEGEND System Reference Manual page 383

Features
Telephone Differences
Multiline Telephones
Single-Line Telephones
Considerations
Feature Interactions
Auto Dial
Conference
Direct Station Selector
2-334 Signaling
When the signal sender presses the programmed Send button, a green LED
next to both the Receive button and the Send button goes on. Both LEDs remain
on until the sender presses the Send button again or the receiver presses the
Receive button.
The visual signal sent is one way; if both users want to send and receive, both
telephones must be programmed with a Notify Send and a Notify Receive
button. Unlike Signaling, Notify cannot be used to see the status of the
telephones nor can it be used to automatically dial the extension.
Both Signaling and Notify require a programmed button. Queued call console
(QCC) system operators cannot use either feature. Feature codes cannot be
used for either feature, and MLX display telephone users cannot select either
feature from the display.
Both signaling and Notify require a programmed button and cannot be used on
single-line telephones.
Signaling can be used even when both users are on the telephone.
Signaling does not activate any external ringing devices connected to the
destination telephone.
To use Notify, two users must setup one telephone with a Send button and
the other with a Receive button. If both users want to send and receive, both
telephones must be programmed with Send and Receive buttons.
A Signaling button and an Auto Dial button cannot be programmed for the same
extension. If a user tries to program an Auto Dial button when a Signaling button
is already programmed, or vice versa, the feature being programmed erases the
previously programmed feature.
Signaling can be used during a conference.
If a user presses a Manual Signal button programmed with the system
operator's extension while making a call to the system operator, the LED next to
the user's DSS button changes from flashing to on steady while the Manual
Signal button is held down.