AT&T MERLIN LEGEND Release 2.0 Programming Summary Manual page 51

Introduction
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.
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Idle States