The Interphone study was carried out for 4 years in 13 countries. It found that use of cell phones over 10 years did not conclusively increase cancer risk.
But the study had some methodology issues and potential conflicts of interest. Read more about the INTERPHONE study.
The contradictory results on glioma development is what prompted the WHO to play it safe and advise headset use with cell phones. In light of recent research on the increased energy use of brain regions near cell phone antennas indicates something is happening, even if it isn't an increased cancer risk.