I found I had this problem Saturday morning -- it had something to do with the way things were routing on AT&T's side. I called them up, and they fixed it up promptly (they first person I spoke to wasn't able to fix it, but they bumped it up to the next tech support level and they resolved the issue within 2 hours).
Have to say, I was VERY impressed with their customer service -- after reading all the consumer reports, I was expecting the worst, and had quite the opposite experience. Call them up, and they'll get you going.
