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Meanwhile, others have begun devising cellphone ring-restriction technology that is legal, at least until further notice. (The F.C.C. maintains no regulations against cellphone-blocking techniques other than jamming, but does not rule out the possibility that such techniques could be scrutinized in the future.)

Bluelinx, based in Charlotte, N.C., is developing a system called Q-Zone (the Q standing for quiet) that uses Bluetooth wireless technology - in transmitters and imbedded into cellphones - to put phones equipped with Q-Zone software into silent or vibrate mode when they are taken into a specified zone.

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Jeff Griffin, Bluelinx's president, said he was trying to sign up wireless providers and establishments like cafes and theaters. He said he hopes to start using the equipment in the next few years. Unlike jammers, he said, his call-blocking system would be optional for cellphone users, who could turn it on or off.

"I was at church some time ago and a lady's cellphone went off and the entire church froze," Mr. Griffin said. "Meanwhile, she couldn't find her phone and was so embarrassed. It's that kind of circumstance we're trying to fix."

A similar system is being developed by Stefan Marti and Chris Schmandt, researchers at the Massachusetts Institute of Technology's Media Lab. Their project, called Autonomous Interactive Intermediaries, uses technology like speech recognition to screen calls to determine when a phone should ring, and even subtle, silent visual cues to replace cellphone rings or vibrations - say, an animatronic rabbit or parrot turning toward you in a room to signal that you have a call.

Mr. Marti pointed out that the technology can always be overridden by users. "People will get nervous if the cellphone starts to make decisions by itself," he said. "It will take some time for people to trust the technology," which could become available within four years, he said.

Mr. Larson of the cellular industry group said that while the industry objected to controls imposed on cellphone ringing, it did not oppose measures left to the customers' discretion. "They are certainly less odious than jammers," he said.

Another means of guarding against cellphone disturbance is the use of detectors, sold legally in the United States and abroad, that sound an alert when a cellphone is present. Zetron, a company in Redmond, Wash., makes the Cellphone Detector Plus, a $449 receiver that sounds an audio alert when it detects certain cellphone frequencies. The model, about the size of a thermostat, flashes a red light, beeps and plays a recording that urges people to turn their phones off. The devices are useful for hospitals, said Vaughn Entwistle, who edits Zetron's company newsletter.

An Israeli company, Netline, makes a detector called the Cellular Activity Analyzer, a hand-held device that is used to monitor and detect cellular communication activity in a given area. (It is offered at www.netline.co.il or www.spyshops .ca for $2,500.) Other smaller detector models include the RF Signal Detector from Suresafe Technology, about the size of a beeper, which costs less than $100. As with jammers, the larger the detector, the greater its range.

A different approach - by design or happenstance, but altogether legal - is to block cellphone signals through construction techniques. (An F.C.C. spokeswoman said the commission had no regulations dealing with building materials.) Like most cellphone-blocking methods, many of these ideas were developed long ago for military and espionage purposes, said Bill Sewell, senior vice president of DMJM Technology, who has spent years designing radio-secure areas for the United States government.

Mr. Sewell said the methods used by his firm are simple: metal mesh screens tuned to the frequencies of radio waves are mounted inside the wall. They are also inexpensive, at about $15 a square foot, he said.

Like Mr. Sewell, Deborah Chung, the Niagara Mohawk professor of materials research at the State University at Buffalo, has developed construction materials that block radio waves. Dr. Chung's "smart concrete" contains electrically conductive mixtures, like metal or carbon particles, that provide electromagnetic interference.

Her structures are designed for the military and hospitals, she said, but they could be used in other structures to keep cellphone users away. "It certainly would work," she said. "On the other hand, they might not be able to watch TV inside."

Some buildings have the blessing or curse of being cellphone-proof by accident, thanks to heavy walls. An example is the Frederick P. Rose Hall, the new home of Jazz at Lincoln Center, which is scheduled to open in October at Columbus Circle in Manhattan. The building is designed as a box within a box, with two sets of walls - the auditorium wall and a separate lobby wall - to prevent sound from seeping in. This double-thick construction, said Walter Thinnes, the vice president of Rose Hall, prevents cellphones from working in the auditorium.

"This is an unplanned helping hand," Mr. Thinnes said.

Without such help, there is a last resort: personal responsibility. "There are always going to be rude people," Mr. Larson said. "We just hope they will learn to turn their cellphones off at the right time."

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