This has been widely and wildly inaccurately reported and now the proposition has been turned upside down to make it sound like a dire warning and "no study" to back it up.....just more irrelevant interference by the lefty WHO crackpots. I've spent some time at WHO and they aren't crackpots, nor was there a study, but a review of scientific papers and it wasn't a dire warning...it was the mildest level of concern/interest.
I would opine that any device that is in such widespread use (6 billion of the things by one count) and that number making it the single most common electronic item on the planet...well it is not horrible that someone takes a look at it. Radio frequency emitters are and have unusual properties just in case you didn't know it. In the presence of a strong electromagnetic field, you can cut a raw steak with the handle of a spoon and the heat searing the edges shut. It is extraordinarly useful in a cutting/searing application.
When you hold cell phone up to your ear for hours on end it does transmit a radio frequency and does create, albeit small, an electro-magnetic field. Everyone is pretty sure it is harmless but some papers published here and there have demonstrated a hint of a link.
That's all there is to it. Don't get crazy.