I, too, have run into the Google Voice no-answer for IVR issue found here: https://michigantelephone.wordpress.com/2011/03/22/google-voice-incoming-calls-broken-with-asterisk-1-8/
This patch allows an "IVR Mode" (under Advanced Settings) that will always pick up the phone, allowing an IVR to be the destination for an incoming call, and allow GV Voicemail to be used if the PBX is offline.