There are several ways to interpret this question.

First, let's look at it from the perspective of the cellphone owner:
For the cellpone to work, it must radiate. The microwave radiation emitted by the phone must reach the cell tower. Furthermore, the emissions from the tower must reach the phone. The trick is to allow this communication to take place, but minimize the amount of radiation that is "wasted" by being absorbed by the user's body. There are several ways to accomplish this.
Increase the distance between the phone and the body. By increasing the distance, the intensity of the radiation is decreased... just like the flame from a candle is hotter close up to the flame and cooler as distance increases.  Using a built in speaker phone is one way to accomplish this. Using a hands-free headset is another. Of course, if you hold the phone in your hand, or in a pocket or purse near your body, you have not reduced your exposure, only transferred it from your head to another part. Naturally, the further the phone is from the body, the less radiation is absorbed by the body. Use an extension cord on the headset if you can, and put the phone down.
Place a shield between the phone and your body. There are several styles of cellphone shield which block the emitted radiation on one side of the phone.  skin-blok Naturally, you will want that shield on the side of the phone that is between the phone and your body. You can use the shield style that goes right onto the phone, or you can line a pocket or purse with a shielding fabric. The shield should be at least as big as the phone, since the entire phone radiates... not just the antenna.
Notice that these techniques reduce the user's exposure. Because the room is still filled with the microwaves emitted by the phone, exposure is not eliminated, and of course there is no benefit to others in the room.
Now, what's so hard about blocking a cell phone signal completely?
Let's say you don't own a cellphone and want to shield your house completely from external signals. Or maybe you have a secure area (such as a hospital, data center, prison, or movie theater) and you want to prevent incoming or outgoing calls. There are many shielding materials you can put on walls and windows. But there is a big problem: cellphones can operate very nicely with only a very small fraction (less than 1 millionth) of a normal signal.
1- Therefore your shielding materials must provide very high attenuation levels. Typically, 80 to 100 dB attenuation materials are required. RF leakage

2- You must control leakage point VERY carefully. Gaps under doors, joints between shield sections, and even pinholes from sewing shielding material can permit these high frequency signals to penetrate. You need to create a "complete" enclosure. Any part that is not shielded is a leakage point.

A small pouch is not that difficult to make with the proper material. Shielding a whole house, or even a whole room is a more difficult challenge... if you want to completely kill the signal.

(from LessEMF)