For iPhone users, you can read how far you should keep the device away from your body here:

Settings - General - About - Legal - RF exposure

In 1998, I worked for a company that built Nokia phones. It was announced/published in many medical journals that cellphones definitively caused cancer if held to the skin. But the general public/media deemed the benefit greater than the cost.

I've occasionally seen it in the media since then, but rarely.

The rule of thumb is hands free and don't use the in ear headphones as they act as an antenna into your ear.

NOTE: I can not provide any sources for these claims. I was simply part of a company that made them back in 1998.