You could delete the prompts and routing rules from the OBi Auto Attendant then route the call to the AA. The caller will hear silence, but after several seconds it will timeout and the caller will hear "There is no call route available".
If you are interested in any of these, I'll show you how.
I am interested in the AA method. I wonder if it's possible to change the timeout and disable or change the "There is no call route available" recording. Perhaps make a 1 second silent recording to use in its place.