AT&T MERLIN LEGEND Release 3.0 Programming Summary Manual page 40

Idle States
When the telephone is forced idle, the following occurs: multiline telephone
users hear a signal, indicating that the telephone cannot be used; display
telephone users see the message Wait: System Busy; single-line telephone
users do not hear a dial tone.
100D Module Idle
The following procedures can be performed only when the 100D module is
idle:
Specify board type.
Specify frame format.
Specify board signaling format.
Specify board suppression format.
Specify board facility compensation.
Forced Idle Reminder Tones
The forced idle reminder tone is a high-low "doorphone" tone—400 ms of
667 Hz tone followed by 400 ms of 571 Hz. The tone is provided under the
following circumstances:
At the extension, to remind the user that the system or the extension is
in the forced idle state
At the programming console or at a PC running SPM, to remind the
system manager that the system (or at least one extension) is in the
forced idle state because of administrative activity
In Release 1.1 and higher of the communications system, forced idle reminder
tones occur every 20 seconds. You can adjust the volume of these tones with
the volume control.
1-14 Programming Basics