Google Voice has its own voicemail system. It is not designed to be used as a bare-bones VoIP service with your own answering machine. Use Google Voice's voicemail, or don't use Google Voice.
You cannot make 100% of inbound calls go to an answering machine. For example, if you are already on a phone call, and another one comes in, and you don't use call waiting to pick up that second call, it will be sent to Google Voice's VM. You cannot disable this behavior.