There are tens of thousands of people on the line!
Well there's your problem. That's way more traffic than AT&T can handle. Livestream your meeting so more people can tune into it. I don't know of any system that can handle a teleconference call with tens of thousands of people connected at once.
As a former IT guy, we had to tell our customers to stop having massive teleconference meetings because it would overload our network and drop people from the call.