It turned out that the "error" was that out of the three pairs going into my home, only one could be provisioned because two were hooked up to bad lines going from a point under my sidewalk (where the neighbor's lines also hook up) to the CO. AT&T told me unofficially, since I am not their customer and they weren't even supposed to talk to me about it, that if I had the service, they would simply switch pairs to a working one. Since Sonic was the customer, they were entitled to the same level of service as any other customer. But Sonic was a dead end. AT&T couldn't put in a work order based on what I said. And Sonic kept telling me that there was nothing that they could do, because the problem is at AT&T's end. It was, but Sonic had the right to ask them to fix it. It was like talking to a brick wall.
I finally have AT&T's database corrected (not thanks to Sonic or official channels) so I was able to order the upgrade through Sonic. AT&T should show up, find a problem with the line quality, which they can fix by swapping out the connections, and they can go from there. The only question now is whether sticking with Sonic was a mistake, because they refuse to fix problems (not just this) or take responsibility for what's obviously at their end.