The phone is always in receiving mode, meaning that it constantly picks up the radiowaves that are all around us all the time. It also emits via a small weak transmitter—this is how it finds the nearest cell tower—but I am not sure how often it does that. When you talk on it, of course, it is constantly transmitting.
Another source of EM radiation would be the electric cords all over your house. If an electric cord is receiving power, it is also emitting at a low frequency.
The EM spectrum is all around us. You really only need to worry about being exposed to the high frequencies (UV and above) that can damage DNA, or about extremely intense exposures at lower frequencies, which can burn you.
Thanks.
The registration with the cell tower happens once...when it comes in range. As the phone moves from one segment (there are 6) of a cell tower's area of coverage, the phone re-registers. This happens at far below full power of the phone.
When a call is made of received, the phone briefly transmits at full power which is .8 watts. The cell tower then tells the phone which one of 8 different power levels to use...based on distance from the cell tower.
As a comparison, the old, installed-in-the-vehicle phones with an external antenna used to have full power capability of 3 watts.