Does anyone know of some evidence that illustrates that cell phone companies knew that cell phones caused brain Tumors and then didn't notify the public?

For instance I heard in the early 90’s that radioshak was having people who bought cell phones sign a legal waiver that they knew of the risks of these cell phones…which shows prior knowleadge on the companies part. Did anyone experience a waiver such as this at radio shack or elsewhere; or experience something else that illustrates pre-knowleadge of the brain tumor cell phone nexus?