How can cell phone causes brain tumor ?

Probably doesn't: Several studies have been done to study this with the thought that the magnetic field and radiation of cell phones cold cause brain tumor. So far no definite cause and effect has been proven.

Cell phone use: Cell phone radiation is low-frequency & low energy. It is actually nonionizing radiation. Unlike high frequency wave ionizing radiation, it should not cause cancer. Cell phone radiation was however, shown to ^ risk for developing schwannomas in rats & gliomas in other animal studies. REF: https://www.mskcc.org/blog/do-cell-phones-cause