Some will say it's POTS. I did a quick search on Google and found this:

Quote:
With respect to communications, the [9/11 Commission Report] concludes:”Almost all aspects of communications continue to be problematic, from initial notification to tactical operations. Cellular telephones were of little value….Radio channels were initially oversaturated….Pagers seemed to be the most reliable means of notification when available and used...”

- 9/11 Commission Report. Chapter 9, p. 315.

http://aquis.aquiscommunications.com/products/wireless-messaging.html
(no affiliation)

Pagers!
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If you're reading this, it's too late.