MY Gv sometimes answers with the generic "The Google Voice Subscriber is not available" rather than my custom greeting. All my settings are correct so I am wondering if anyone else is having this issue?
Have you looked under "Groups & Circles" to see if there are any groups configured to use the "System Standard" greeting?
I'm assuming you've already set your custom greeting to be the default greeting under Voicemail and Text>Voicemail Greeting?