Hi Ken:
I'm not the expert on the auto-attendant, but I am the GV guy. When you call your GV phone number from some other phone (be it your forwarding phone, or some unrelated phone number), you're calling the Google Voice system, not your OBi. So, GV is going to handle the call itself, according to its own rules. When calling from a forwarding number, GV will (depending on a GV setting) either send you directly into your voicemail administrator menu, or ring your GV number first, and require you to press the star (*) key to barge in. The OBi is just another forwarding destination, as far as GV is concerned, emulating a computer logged into your Google account using Google Chat.
By contrast, if you have a SIP service provider configured, the call is terminating on your OBi, which answers the call, and can the OBi can thus pre-process it and decide if it should send your call to the auto attendant or not, based on caller ID.
Perhaps one of the other frequent posters here will explain the attendant behavior in more detail, and how to deal with this, but you may need to get a SIP DID (inbound phone number) to make it work.