How reliable is Talkshoe for large audiences?

I am organizing a series of Talkcasts featuring some high profile spiritual teachers. We may well get telephone audiences in or around the max of 250 and webcast audiences in the thousands. Can someone reassure me that the system will be reliable for such big audiences?

Also, what happens if the audience is larger than the Talkshoe capacity? E.g., if more than 250 callers call in, what will happen? Will the 251st and beyond callers get an error message? Will they be told to try the audio webcast? Similarly, what would happen if, say, 10,000 people tried to listen to the audio webcast? Is there a max number, beyond which participants would be shut out of the webcast? (ie, would they receive a message saying the webcast is full?) I want to be reassured that in the event we get a very large audience, the whole system won't crash.