Call screening in Google Voice is off, so unfortunately that's not the answer. To clarify, this is actually the "find me" function of Voipo and happens also when i pick up a call - I have to hit "1" on the mobile keypad to accept the call. Very frustrating because this function worked well on my previous voip provider's service - if i couldn't pick up the call it would go to Google Voice and then i'd get SMS notification and email with transcript (still a work in progress, Google!).
I reset my settings to use Voipo voicemail instead and today I got a email notification but the SMS notification didn't come through.