I did the following (a 7/7/2003 post from TouchToneTommy) and simply get the voicemail message for the operator now.
Step-by-Step of what I did:
Menu
System Programming
Exit
Extensions
More
Grp Calling
Line/Pool
791 (the unused "delayed-call handling" group)
Enter
[Enter each line number, or the pool number that they are in]
Enter after each entry
Back
Back
Grp Calling
Overflow
791 (the unused "delayed-call handling" group)
Enter
770 (or whatever the Voice Mail calling group is)
Enter
Number Based Overflow
Backspace out the current number
99
Enter
Time Based Overflow
Backspace out the current number
[the number of seconds before the AA should answer]
Enter
Now create a mailbox by logging into the system administrator's mailbox. Press 4 for mailboxes, press 4 to create a mailbox, dial 791#, answer 15# for the class-of-service, I think you still have to give it a name (this part is from memory, just follow the prompts)
Sounds like you have the Night Service button set up right, but here is the sequence:
System Program
Exit
Night Service
Group Assign
Calling Grp
[extension number of the Night Service Operator]
Enter
770 (or whatever the Voice Mail calling group is)
Enter