I am guessing that, by "auto attendant", you mean the auto attendant of the company you are calling (such as "For reservations, press 1, for awards travel, press 2...", right? The OBi device has its own (optional) auto attendant, so we need to be clear.
If my guess is right, then this is a quirk of the way the airline is using their auto attendant (IVR) system. They don't actually send a "call answered" signal until you press a button and your call is ... answered by a human. They save a ton of money this way, because toll-free numbers cost them a per-minute charge. This causes Google Voice's system to think that it's a no-answer call, so it gives up and ends the call.
Use some other phone service to call that number.