I don’t talk on my cell phone very much but I have worn it on a belt clip for years. I wonder if the danger, if there is any, would only be if you were making a call?
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.