The frequencies required for cellular phone communication are between 100 MHz. to 2.5 GHz!!.... and newer cordless phones operate at frequencies up to 5.8 GHz. 
Until 2004, the health risks attributed to cellular phone radiation were believed to be caused by the cumulative effects of these frequencies generated by a phone, which is referred to as near field.
All phone manufacturers and the World Health Organization warn consumers not to use any shielding product that claims to absorb radiation.
When an absorption based shielding material is used, the phone must compensate for a weaker signal by increasing its output power, thus increasing its radiation emissions and battery consumption. Further, all absorption based products, such as sponges, deteriorate over time and leak when they become saturated.
People believe that radiation is emitted from a phone's extendable antenna mast or earpiece, however it is the radio system (transmitter and receiver) that generates more than four times the amount of radiation as the portion of an antenna inside a phone.
The amount of radiation emitted by the radio system is contingent upon its design and the strength of the signal received through the antenna from the base station. At least 40% and up to 60% is absorbed by the head and neck. 
