An F.C.C. spokesman, Clyde Ensslin, declined to comment on the issue or the case in Maryland.

Cellphone carriers pay tens of billions of dollars to lease frequencies from the government with an understanding that others will not interfere with their signals. And there are other costs on top of that. Verizon Wireless, for example, spends $6.5 billion a year to build and maintain its network.

“It’s counterintuitive that when the demand is clear and strong from wireless consumers for improved cell coverage, that these kinds of devices are finding a market,” said Jeffrey Nelson, a Verizon spokesman. The carriers also raise a public safety issue: jammers could be used by criminals to stop people from communicating in an emergency.

In evidence of the intensifying debate over the devices, CTIA, the main cellular phone industry association, asked the F.C.C. on Friday to maintain the illegality of jamming and to continue to pursue violators. It said the move was a response to requests by two companies for permission to use jammers in specific situations, like in jails.

Individuals using jammers express some guilt about their sabotage, but some clearly have a prankster side, along with some mean-spirited cellphone schadenfreude. “Just watching those dumb teens at the mall get their calls dropped is worth it. Can you hear me now? NO! Good,” the purchaser of a jammer wrote last month in a review on a Web site called DealExtreme.

Gary, a therapist in Ohio who also declined to give his last name, citing the illegality of the devices, says jamming is necessary to do his job effectively. He runs group therapy sessions for sufferers of eating disorders. In one session, a woman’s confession was rudely interrupted.

“She was talking about sexual abuse,” Gary said. “Someone’s cellphone went off and they carried on a conversation.”

“There’s no etiquette,” he said. “It’s a pandemic.”

Gary said phone calls interrupted therapy all the time, despite a no-phones policy. Four months ago, he paid $200 for a jammer, which he placed surreptitiously on one side of the room. He tells patients that if they are expecting an emergency call, they should give out the front desk’s number. He has not told them about the jammer.