SIP access to Google Voice Numbers is no longer available. As it was reported and discussed by Todd Vierling, Alok Saboo, and Dan York over the past couple of days, access to Google Voice numbers WAS available in the following format:
SIP:+1{insert GV number}@sip.voice.google.com
This capability is no longer available. The domain is no longer accepting SIP traffic. See here for unanswered INVITES.
Hopefully, it was a sign of things to come.