Today I got notification that I missed a call coming through my Google Voice account. I checked Google Voice and everything was setup correctly; I had also noticed that the Google Voice app on my iPhone didn’t notify me of an SMS that I received the other day which was strange.
Thinking that things may have broken after I ported my cell number to AT&T last month, I deleted my cell number from Google Voice and added it back. While I shouldn’t have been surprised, I was when that simply fix made the forwarding work again and notifications started working again in the Google Voice app.
So, if you have trouble with Google Voice, delete the forwarding number(s) and re-add them.