On the incoming line get it to ring the reception extension first.....
on the reception extension set the 'On no answer' tab to 10 seconds to divert to the ring group
on the ring group set the 'On no answer' tab to 10 seconds to divert to the Auto Attendant.
Just becarefull though because 15 seconds of ringing before and auto attendant picks up is a long time....
People might get bored and simply hang up or because a auto attendant answers they may think something it wrong with the phone system and thats why it too so long to answer