(Un)fortunately, jamming phones in the US is much harder than jamming phones in Europe. CDMA, the dominant cell technology here, is based on military tech that was designed to resist jamming.
You can keep GSM from working (probably also TDMA, iDEN, and AMPS) by jamming certain specific frequencies, but to keep CDMA from working, you’d have to jam the entire cellular and PCS frequency range.
Not true, actually.
For all these technologies, you’d have to block all the channels (frequencies) in use at that particular location, because even though TDMA (and similar) use only a small “slice” of the range per call you wouldn’t know which are being used
at any given time for a particular call. So to block all calls in, for example, a theatre or exam room, you’d have to block all channels being used by the sites serving that area for all the carriers licensed in that area.
This wouldn’t necessarily be the entire frequency range, because not all frequencies (channels) are used at every site. (Adjacent sites use different freqencies and phones/calls jump between them - for CDMA, TDMA, GSM, GPRS, ESMR/iDEN, analog, whatever - that’s how you get hand-off.) But you would need to know what frequencies are being used by each carrier in that area, and because this changes over time (as sites area added), and as they’re not very forthcoming with this information (many of them consider it “proprietary”), it would be difficult to establish, so you’re back to blocking the entire licensed spectrum slice.
Of course this doesn’t even get into the unlicensed services like WiFi or the narrowband PCS (AKA paging) services like Blackberries use.
Well, my understanding is that certain channels are reserved for control data, and those channels must be used to place or receive calls, text messages, voice mail notifications, etc. By blocking only the control channels, you could keep people from placing or receiving calls inside the theater, although they’d still be able to start a call outside and bring it inside.
In CDMA, jamming the control channels is no easier than jamming the voice channels, since all CDMA “channels” are spread out over the same frequency range. But in GSM, TDMA, iDEN, and AMPS, the control channels live at a fixed set of frequencies.
Am I wrong?
Hmmmm…I was thinking even CDMA control channels were at set frequencies (in that they’re “set aside” like for the other technologies), but now you’ve got me wondering. (Thanks for that, BTW. I can always learn more.)
If I get a chance tomorrow, I’ll look into it.
Would a Jacob’s Ladder generate enough EM output to jam cellphones &/or text messages?
If so–decorative and useful! 
What the hell’s so expensive about a Farady cage? Line each auditorium with some metal screen and ground it. Screen is cheap. This can’t be more expensive than the red velvet stuff they put up there now.
What security risk? Step out of the auditorium, and there’s your signal. What’d we all do before cell phones, blackberries, and pagers?