AT&T MERLIN LEGEND System Reference Manual page 384

Features
Do Not Disturb
Group Calling
Messaging
Multi-Function Module
Privacy
Queued Call Console
Transfer
Related Features
Signaling cannot be used when the destination telephone user activates Do Not
Disturb.
A Signaling button cannot be programmed for a calling group.
If a display telephone user presses a Signaling button only to send an audible
signal to a telephone with a posted message, the posted message at the
destination is not shown on the display. However, if a display telephone user
selects an Intercom or System Access button, lifts the handset, and uses the
Signaling button to dial the extension, the posted message at the destination
telephone is shown.
When set for SAA operation, an MFM can receive a manual signal but cannot
send one. An MFM cannot receive a manual signal when set for tip/ring
operation.
Users can program and use Signaling to get through to co-workers who have
activated Privacy.
Notify and Signaling buttons cannot be used on a QCC. However, pressing a
DSS button sends a manual signal to the station associated with the DSS button
in the following instances:
The QCC system operator is "high and dry" (timed out from dial tone on a
Call button or pressed the Forced Release button while listening to dial tone
on a Call button).
The QCC system operator, in a split condition, pressed the Source button
after contacting the destination but did not connect both parties by using the
Join button. If the operator presses a DSS button, a manual signal is sent to
the destination station.
A Signaling button can be used to dial the extension during a transfer after the
Transfer button is pressed. However, Signaling buttons cannot be used to
initiate One-Touch Transfer.
Barge-In
Messaging
Signaling 2-335