Yes Google Voice sends an SMS voicemail notification, to multiple phones that you configured, and that's what I'm currently using in lieu of getting the MWI light unsolicited SIP NOTIFY done.

When Google Voice sends an SMS voicemail notification, the SMS sender number is your personal Google Voice number, and it contains the fixed text "You've got a new voicemail from " followed by a phone number, followed by ". Preview: " and then a transcription of the voice mail (which is sometimes pretty funny but often accurate enough for you to understand nonetheless). So the SMS is easy to verify that it came from Google Voice, ignoring caller ID spoofing.

If VOIPo added a feature where if I configured Google Voice to send an SMS to my VOIPo SIP phone number to activate the PAP2T's MWI light, then that would be an amazing feature for VOIPo to be able to market as a huge differentiator between it and its competitors. I don't think any other VOIP provider provides that.