No, pretty sure I'd show more than half of one bar (flickers between 1 and zero bars) if the tower was busy. Just plain don't get a signal inside the Faraday Cage.
not how it works. You can show full bars and still not be able to do anything with your phone, which is what happens with AT&T in the stadium. Think of it this way, you want to walk across 5th Avenue to Steak Escape, you can see it right there clear as day, but traffic is so heavy you can't get to it. The door is there, there's just 20,000 people trying to get through it all at the same time.
Sprint and Verizon seem to do better in the stadium, but I think that has to do with lower market share than better infrastructure. I would say at least 70% of the people in the stadium have AT&T.