Just figured I would through this out there since I could not find the answer and had came up with a solution :-)

With an Straight Talk (AT&T) SIM card you can do CCF (conditional call forwarding) on a Droid DNA but not with the phone itself. If you try using the code **004*1YOURGMAILNUMBER# you get an error. If you install the Google Voice app it looks as if it changes your number and sets it up correctly, but when you call your number you still get the carrier voicemail. The same thing happens with the Youmail app.

The way I got around it was by using an actual AT&T phone (I would assume an full GSM phone would work as well) and entered the **004*1YOURGMAILNUMBER#. Just swap your SIM, enter the code, swap your SIM back and all done. I did verify CCF before swapping the SIM back to the DNA, and after.

Hope this helps someone else!!