What O.J., Katrina, and 9/11 did to AT&T's network

By far the largest strain on AT&T's network came on Sept. 11, 2001. People from all over the country tried to contact relatives and friends, and placing calls to Washington and New York was a near-impossibility for much of the day. Virtually every point-to-point network connection in the country was overloaded.

The second-, third-, and fourth-highest call traffic days on record were the season finales of American Idol in May 2009, 2008 and 2004, when millions of viewers from across the country texted and called to vote for the winner.

Ironically, the fifth-highest call traffic day was June 30, 2003. That's the day that FCC opened up the Do Not Call registry.

NEXT: Normal call volume