Yes, it is true that usage of cell phones can cause Brain Tumour. Many studies all over the world have shown that extensive usage of cell phones can cause very harmful effects on human minds and reason is that it emits some radioactive waves which harms brain.
Although, many countries have taken many steps to minimize this danger but still it largely depends on usage and other situations like signal strength. Even the cell phone usage can be minimized or can be controlled but what in case of low signal strength?
When you are using a smart phone in area where the signal strength is too low then your cell phone will emit more radioactive waves to search more signals. These wave makes their entry to human brain and causes harm. Although this theory is not a theory accepted on record by officials but it has been believed worldwide.
To reduce the low signal problem to reduce the radioactive waves coming out of cell phone, you can install a cell phone signal booster. It will boost low signal strength to highest band possible and your cell phone will not emit any radioactive waves to search signals.