The legality of the Phone Jammers varies from country to country. In the United States, it is generally illegal to sell, own or use one without the government's permission. The devices are offered for sale on a handful of websites.

Mislan, a former communications electronic warfare officer in the U.S. Army, said law enforcement has "very specific worries" about how cell-phone jammers could be used by criminals.

But even someone looking to do no more than hush an annoying neighbor on the bus could do some harm, he said.

For example, in the Philadelphia case, the jammer could have cut off the bus driver's communication with a dispatcher who was trying to communicate emergency or traffic information. And that's not to mention other folks in the area (aside from the offensive loud talkers) who may have missed potentially important phone calls.

"Who is he to play god with our cell phones?" Mislan said.

Wifi jammer works in much the same way online denial-of-service attacks on websites do -- transmitting a signal on the same frequency as mobile phone calls in the area.