I'm also trying to get U-Verse + VOIPo to work but with no success thus far. I have the U-Verse 2wire RG with the latest 6.1.9.24-enh.tm firmware - there is no setting or backdoor entry to disable SIP. It seems as though this is the culprit.

I have the ATA downstream from the RG, tried both bridge mode and NAT on the VOIPo box but that did not seem to make a difference.

Neither UDP 5004-65000 port range forwarding nor DMZplus mode are working; outbound calls work fine, but incoming calls go straight to voicemail and do not show up in the vPanel. I was pleased with VOIPo's customer service turnaround time; I created a ticket this morning (Saturday) and received a response within 20 minutes! Unfortunately, VOIPo has restricted access to the administrative features of the ATA (Grandstream 502) so I had to wait for them to remotely make changes. They asked me to power cycle all of my devices and that still did not do anything.

Right now I'm waiting for a response to my ticket, but I'm pretty much out of ideas.