Originally Posted by Herb Kornfield

The real reason is that the lines are closed down to allow emergency access to reach each other without having to deal themselves with an overloaded voice network.

Keep in mind that most networks base their call volumes on a standard usage pattern with normal day to day traffic. Just like server MRTGs.....

When an event like today's occurs the networks are flooded with a much more massive call volume plus a much greater call duration time than the usual traffic patterns allow for in their tolerances.

Instead of having the infrastructure crash hard and burn for all people ( including the emergency folks) the powers that be merely shut down the voice network to allow emergency traffic to pass and restore to normal later when a few hours pass from the main impact point of the event.