I can't make much sense out of the wording of your first post.
Google Voice is an inbound call forwarding system. If you have an inbound Google Voice number, and someone calls that number, then it will ring all the forwarding destinations you have added and enabled (check-marked) on the Phones tab of Google Voice Settings (linked above).
GV will simultaneously ring all forwarding destinations. Whichever one answers the call first (answered either by you, a human, or by that number's answering machine/voicemail), during the ~25 second ring period wins the race and gets the call.
The "Google subscriber is not available" message is simply the default Google Voice voicemail greeting. That's normal, until you record your own greeting.