In the 5000 this is actually fairly easy...
Create a new Hunt Group (I will use 2500 but whatever fits for you is fine), then create a new Phone Extension List (P0005 or again whatever fits), and add that in the HG 2500's Members list, and set the Recall Time to about 20 seconds but you can play with this as needed, and now add the AA (2900) as the Recall Destination of the HG 2500. In the Trunk Group of the 3 analog lines, change the Day Ring destination to 2500 and the Night Ring destination to 2900, and you are all done!
Note that using an Auto Attendant is likely not the best solution, a Call Routing Announcement is usually the better way to go.