I don't remember ever being able to do that on Nortel phone systems.
Which again, if I am remembering correctly is one of the things I don't like about Nortel Mail products.
To get the line to be answered by the auto attendant you have to assign the line to be answered by the auto attendant.
Others here may know a way around this, but I haven't found one.
Generally the lines are set up so that after x amount of rings the auto attendant will pick up, or it is picked up right away always. I know there are several tricks to have the AA answer right away and hand it off to a live person during day time, such as recording silence in your day greetings and the setting up a transfer to the operator, but I have found no provision to tell the Auto Attendant to answer the line only during certain hours. (Only the greeting tables are time sensitive)
Then again, I haven't faced this scenario for awhile so I may have forgotten something...
I Suppose you could send the call to the voicemail DN with out a mailbox and then record a custom Auto Attendant greeting, but you would lose some of your routing capabilities. However, people could dial extension and go to the company directory.
VBut I think the way to do that would not be by assigning the voicemail DN to the night ring group but forwarding a phone with the line ringing on it to VM