I'm guessing you have both an OBi phone and your cell phone linked to GV with incoming calls forwarded to both. If so, what's likely happening is that when you turn your cell phone off, your carrier starts forwarding incoming cell phone calls to it's voicemail immediately. To GV, this looks like the cell immediately answers and the calling party is left connected to your cell providers voicemail.
One solution is to leave your cell phone turned on. GV sets their timer to go to VM to 25 secs and most cell providers set theirs to 30 secs, so GV usually wins out.
Another possible solution would be to enable Call Screening. With Call Screening enabled, you'll be asked if you want to accept the call and you'll need to TouchTone a response. Until GV gets that TT, it won't transfer the call, and your cell phone VM doesn't normally send a tone, so you should be able to still pick the call up on your OBi.
This said, Google has been making a lot of changes lately, and I haven't tested any of this in the post-XMPP era. However, I'd bet this is what's happening.