Jammers, however, are not the only tool for battling contraband cellphones. Several companies are marketing cellphone detection as a smarter alternative to jamming. The principle is straightforward: instead of blocking signals, prison authorities can use a network of sensors to detect cellphone transmissions, measure their use and triangulate their location.

I spoke recently to Terry Bittner of ITT Corporation, which markets a system called Cell Hound. It's currently installed in some state and federal institutions. According to Bittner, a system like Cell Hound is more of an intel tool. Illicit phones are often hidden with other contraband -- and detectors can not only locate phones, but track the patterns of the callers. "You need to study the habit of the phone -- who uses it most of the time, how it got in there," he said. "They will never store the phone in a cell where they use it. They rent it out to other inmates. In some cases they won't use the phone." Cell Hound scans for the most common cellphone radio-frequency signatures in North America; a central server then maps the location and gives a visual alert on a corrections officer's workstation.

ITT is not the only company in this game, however. Israeli prisons also have a major issue with contraband phones (pictured here); Israeli electronic warfare firm Netline also markets a cellphone detection system with a central control. Cellphone detection could potentially have other applications, too: Detecting illicit cellphone use inside secure conference facilities, or to aid a network in pushing marketing messages or alerts to mobile phones.

Typically, law enforcement and intelligence agencies don't like to advertise what kinds of tools they use to counter or detect illicit communications. ITT, for instance, initially marketed Cell Hound in a very low-key fashion; but pending legislation in Congress, as Bittner described it, forced the company to "come out of the closet" with its technology.