I've used google voice for years and it has worked great. Recently I bought a BlueAnt hands-free so that I can talk while I drive, and I found it has problems.
I configure google voice to pronounce the name of caller when I receive a call, which allows me to decide whether I want to take the call by pressing 1 or 2. This a great feature I like to keep. However, with my BlueAnt handsfree, I will have to fumble with my cell phone to press the number, which defeats the purpose of having a Handsfree.
I don't know the extent of the feature set of the BlueAnt device you're using, but here's my assessment:
You've got Call Presentation/Screening enabled with Google Voice (this is the way I use mine as well), and when you answer a call you have to be able to access the dialer of the phone so you can acknowledge with a key press. How this would be done with the BlueAnt is the trick. Can you just say "dialer" and it will provide that opportunity, or what limitations does it have?
Your assessment is totally correct. Blueant doesn't give me a choice to dial a number via voice command in the middle of phone call, nor does my cell phone.
I hope google can modify their call presentation/screening - if there is no number pressed within a few seconds, then the call will automatically be answered, and if the receiver hang up the phone, then the call automatically goes to the answering service.