
The traditional telephone system has several well-known difficulties
dealing with wide-spread emergencies.  However, traditional reliability
numbers don't tell the whole story.  Emergency planners need to be
careful when hearing that the telephone system is 99.999% reliable.
The quoted figures exclude performance during periods of extreme stress.
While other technologies may appear to have less reliability, you need
to look at performance at the limits of the curve.  Other technologies
may have better performance characteristics at the extreme limits.

Likewise, you need to be careful about applying solutions developed for
the telephone system to other types of networks.

http://www.nytimes.com/2001/03/08/technology/08TOWE.html

  While fiber optic technology proved invaluable, other technology fell
  short. 

  The cellular telephones that controllers had come to rely on were useless
  after the earthquake, and land lines were jammed. The walkie- talkies they
  had kept after converting to cell phones were few and old. 

  "We discovered that reverting back to basic two-way radios was what we
  really needed," Mr. Owen said. "If there is a next time, we'll have better
  two-way radio communications."




