Over the course of a year participants had to fill out a questionnaire about their mobile phone habits, as well as answer questions about their psychological and physical health.  They then completed a series of computerized cognitive tests. Röösli said a unique feature of the study was the use of phone user data from mobile phone operators. That meant for every call made by the participants, the researchers "knew on which network it took place and how long it lasted." 

The study, published in Environmental Health Perspectives, found that one year's worth of exposure to mobile phone radiation could have a negative effect on the development of memory performance in specific brain regions in adolescents. Here ‘exposure' almost exclusively refers to phone calls. "80 percent of the absorbed radiation comes from holding the phone to the head," Röösli noted.

Interestingly, they found the brain's memory function was more vulnerable to the negative impact of radiation when the phone was held to the right hand side of the head. That's where the areas of the brain related to memory are located. You can use loudspeaker or headphones if you're worried about radiation effects on your brain. 

...As for other smartphone uses – sending text messages, taking photos, using apps –these involved "almost no [radiation] exposure to the brain."