And the word of the moment is, "usually"

[Jamming cellphones] is an increasingly common technology, with federal agencies expanding its use as state and local agencies are pushing for permission to do the same. Police and others say it could stop terrorists from coordinating during an attack, prevent suspects from erasing evidence on wireless devices, simplify arrests and keep inmates from using contraband phones.

But jamming remains strictly illegal for state and local agencies. Federal officials barely acknowledge that they use it inside the United States, and the few federal agencies that can jam signals usually must seek a legal waiver first.

[From Local Police Want Right to Jam Wireless Signals - washingtonpost.com]