AT&T MERLIN LEGEND Release 2.0 System Programming Manual page 48

Programming Overview
100D Module Idle
The following can be done 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
Forced idle reminder tones are provided in the following situations:
At the telephone, to remind an extension that the system or the extension
is in the forced idle state.
At the programming console or SPM, to remind the system manager that
the system or at least one extension is in the forced idle state because of
administrative activity.
This tone is a high-low "doorphone" tone (400 ms of 667 Hz tone
followed by 400 ms of 571 Hz tone).
In a Release 1.1 or Release 2.0 system, all three tones occur every 20 seconds.
You can adjust the volume of these tones with the volume control.
Idle States 1-21