CALEA is an acronym for Communication Assistance for Law Enforcement Act. It is a law passed by the U.S. Congress that requires telecommunication carriers to build wiretap capabilities into their systems.
On one hand, it is meant as a measure to allow law enforcement to maintain their current ability to perform wiretaps.
On the other hand, it both makes wiretaps much easier to operate, and allows huge increases in the number of wiretaps that can be performed.
As of 1998, it is not being enforced.