The Horror
It's not that cell phones are intrinsically evil per se. It's that they are always one-step away from providing help to our hapless agents, whom I'm trying hard to terrify. Cell phones are umbilical cords to civilization.
The good news (or bad news, depending on your perspective) is that cell phones can be worked around. For more information, see Darren MacLennan's Disconnectia in Worlds of Cthulhu #3. Here are the ways I've made cell phones fail in my campaign:
Radiation
By radiation, I don't mean actual radiation (although I've used that too). Radiation can be some mysterious yet scientific explanation, like solar flares, bad weather, too much metal in the area, etc. In short, it means that due to some natural phenomena, cell phones don't work. The cell phone will work again once the agents move out of the area or the weird weather ends, which makes this the easiest and most convenient excuse to make a cell phone not work. It's temporary and the cell phone still works after the scenario is over.
I've used this excuse countless times. Radiation in Yucca Flats, terrible blizzards, you name it, I've used it. Unfortunately, it's become something of a cliché for the players, who roll their eyes whenever their cell phones don't work.
Isolation
Sometimes, you're so far out of touch that there are no cell phone towers in the area. We've all been in place where we've inexplicably lost cell phone service. Sometimes the towers are down, sometimes the towers aren't working, and sometimes there's no towers at all.
Anyone who has lived through September 11 knows that cell phones can rapidly become useless during a crisis that affects a large group of people. The towers simply can't handle the traffic. And if the crisis is bad enough, towers might be taken out completely. If there's a power outage or a surge, the towers might not even be turned on.
This excuse is easier for players to swallow because it's a direct output of other circumstances. It also means that whatever situation they're dealing with is REALLY bad, bad enough to affect cell phone service. They're usually too preoccupied to question why their cell phones aren't getting a signal.
Supernatural
It's hard to call other people when you're in another dimension, in another time, or on another planet. Generally speaking, players won't even try to have their characters use cell phones in these situations because it's obvious they won't work. What does cause confusion and can add to the feeling of isolation is when cell phones SHOULD work but don't, because things have changed but the characters aren't aware of it.
I've used this a few times, but unless the supernatural somehow manifests in the call itself, like odd whispering or strange noises, it's the same effect as the cell phone not working. I've had the Mi-go try to contact the agents through their cell phone with eerie buzzing, which worked well. But this kind of effect, due to the rarity of supernatural events, should be used sparingly.
Misdirection
Never mind the cell phone not working. The most simple reason calls don't get through is the person on the other side doesn't pick up. Barring that mundane reason (presumably someone calling 911 will get through), it's also possible that the person on the other line simply won't act in the way the character wants them to act.
For example, Agent Blade called his ex-wife and tried to warn her that she was being stalked by a serial killer. She of course, didn't believe him and accused him of being drunk. So Blade still did the logical thing, but because of the circumstances, his call had no effect. Characters branded insane or criminals might not have the luxury of simply calling the police.
It's also entirely possible that cell phone calls can be manipulated. The cell phone might go to the wrong person, or an impostor might pick up. My personal favorite is an old movie trope: characters in an investigation who call a cell phone hear it ringing nearby, a sure sign that the call recipient is already dead. I actually had this happen during the Puppet Shows and Shadow Plays scenario, when Blade ended up calling one of the victims. I couldn't have planned it better if I tried.
Ultimately, communication – and this goes for emails, cell phones, video cameras, and the web – are only as good as the people using them. You can manipulate both the message and the medium to snip the lifeline that we've all become so accustomed to.
Lesson Learned: Cell phones are your friend. Instead of making them fail, consider creative ways to use them to heighten the tension.
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