I had an interesting issue trying to enable this with my Google Voice number.

First, I had to disable call presentation so the call would go through -- I added the calling number to my Google contacts and created a special group for automated calling systems. I changed my group settings to disable call presentation for those callers. Now I get the verification calls.

Now I have a different problem. Most of my attempts have generated a PIN containing the number 4. GV intercepts DTMF on incoming calls, particularly using 4 to begin recording the call, so the nice Voipo lady tells me the PIN is invalid. I tried several times until I got a PIN with no 4 (122182), but in that case, it seemed as if GV didn't pass any DTMF, as I just received the initial prompt every few seconds.

Looks like GV needs a way to pass-through DTMF on certain incoming calls.