How Cell Phone Signals Work
A cell phone's reception is dependent on the signal it gets from nearby cell phone towers. The closer the tower, the stronger the signal. While the cell phone signals can penetrate most materials easily (a bit like Superman's X-ray vision), they can be stopped by thick areas of a solid material, such as metal or rock. You may have noticed, when driving in areas with mountains and cliffs, that your cell phone signal is intermittent. That's because, at times, a portion of the mountain or cliff may be in between you and the nearest cell tower, thus effectively cutting off the signal from reaching your phone.
Elevators are usually composed of thick sheets of metal and electrical components: not easy stuff for a cell phone signal to work its way through. It's not impossible, of course, but the signal must be strong in order to make it through that conglomeration of stuff and still have any strength left to provide reception for your cell phone. Additionally, elevators are usually placed in the central part of a building. It's not that difficult to get a cell phone signal when you are walking around the perimeter of a building, or in an office constructed of thin (non-metal) walls. An elevator is a different kind of place, however, and the thickness of the walls, the material used, and the location inside of a building all combine to significantly weaken any signal trying to reach your phone. You may have noticed the same type of effect when you try to use your cell phone in a fully closed in basement or when driving through a tunnel.
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The solution to the cell phone-elevator problem is simply that you need stronger reception. That's why your cell phone might work in some elevators, but not in others. The closer you are to that tower which is transmitting the signal, the greater your chance of having reception even inside of an elevator. There are also "signal boosters" available which claim to reduce static and increase your cell phone's ability to receive signals. Whether or not these signal boosters actually work is still up for discussion.