Main Day and Night greetings are recorded from the admin mailbox 999, or 9999 depending on you settings for digit length.
The default passcode is 8642
4 then 1 then 1 for day
4 then 1 then 2 for closed (night)
There is an option to enable whether the voicemail follows the day night setting of the phone system (recommended) or follows a schedule set up thru admin.
For ACD RADs, things are much more complicated.
You need to dedicate ports for rads (not part of main group)
Warning: Make sure you DO NOT use the last or first ports on the Voicemail for RAD's (take from the middle somewhere.
These ports are then assigned to a RAD Hunt Group
The Rad Ports need to be set to play a RAD set of greetings
The RAD set of greetings get assigned a single or group of messages to play.
The RAD greetings are recorded thru the Admin mailbox as well
Option 8 I believe.
From there, what greeting gets recorded depends on the settings above.
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Occam's Razor - All things being equal, the simplest solution is the right one.