Follow-up:
Yesterday I submitted a ticket and braced myself for the inevitable talking in circles with a customer service rep who didn't really read the ticket and would want me to try everything I already tried before escalating it. It didn't happen. Instead, Gerard J cautioned me not to hit the reset button while they researched, and later Erik H mentioned that they are shipping a new ATA configured with both numbers I subscribe to. Done! That easy.

I'm guessing something fried in the HT502. I'd really like to know what and if there's anything I can do to prevent it from happening again. But overall, I'm actually impressed by how quickly this got taken care of. I suppose I shouldn't be impressed with customer service reps actually performing customer service. But I've been run around by so many other companies that I find it refreshing to describe the problem and have someone actually read the ticket and respond to what was written.

Thanks VOIPo.