You assign the appropriate ring group to the Target line, not the PRI channels. Since most lines default to Ring group 001 - I remove any ringing sets from that group. I put the voice mail DN into a different ring group ie. 004. Assign ring group 004 to the target line.
Make sure your control set for the target line is the set that will have the night button. Any lines you do not want to be controlled by the Night button, make sure that you change their control set to None or to a different DN (if you are setting up multiple service modes).
Hope this is clear and helps.