This How-To covers setting up a Hunt Group to Overflow to an Auto Attendant.
I have personally tested these steps on R9.1, but they should also apply to releases below this.
1: Create your Group and Auto Attendant
Open the system’s configuration and create the Group the calls will go to first and the Auto Attendant that you want to forward the calls to. Turn off Queuing and Voicemail on the Group.
2: Create an Auto Attendant Short Code
Create a new Short Code as follows:
Code: This has to be a number and cannot contain * or #
Telephone Number: AA: followed by the name of your Auto Attendant (case sensitive)
3: Create a Dummy User
Create a Dummy User and setup the Forwarding tab as follows:
Forward Number: Set this to the Short Code you created in Step 2
4: Create an Overflow Group
Create an Overflow Group and add your Dummy User. Ensure that the Ring Mode is set to Sequential, otherwise this won’t work.
5: Add the Overflow Group to your Main Group
Add the Overflow Group into the Main Group’s Overflow tab and set the Overflow Time, Mode and whether you want it to Immediate Overflow if there are No Active Agents or not.
You should now be able to call the Main Group and after the No Answer/Overflow time is reached, it should now go to the Auto Attendant you have created.