I am guessing that Nettalk is giving False Answer Supervision. To test, dial your Nettalk number (not your GV number) directly from your cell phone. Don't answer the Nettalk line. If working correctly (no FAS), your cell phone should not show "connected" and start counting call time, until Nettalk voicemail picks up.
If it shows connected too soon, a somewhat cumbersome workaround is to turn on Call Screening in GV (Settings, Calls tab).