If there's no ring in destination, nothing will ring. Sounds like what you want to set it to is MULTIPLE RING IN...go to that trunk group, open MULTIPLE RING IN and DAY (or night). Right-click in the right pane to add extensions. Then under the trunk'group's main screen set DAY RING IN TYPE or NIGHT RING IN TYPE to MULTIPLE. This is plain old fashioned key system ring-all-the-phones stuff.
To take it to the next level you could instead set up a hunt group with queue announcements and recall to auto attendant.
