Such studies have been done numerous times. You need to consider proper blinding to get it right, but it's not too hard. But if you have people who will claim

Such studies have been done numerous times. You need to consider proper blinding to get it right, but it's not too hard. But if you have people who will claim that every study contradicting their belief is a conspiracy by the phone industry that doesn't help a lot. It's like homeopathy: You can throw as many studies as you want at the believers. They don't believe in science, therefore it's not working.