Could keeping my phone in my bra eventually cause breast cancer?

No. There is no known relation. And the phone emmits nothing that can ionize molecules that could lead to cancer. I would however not depend on what we don't know and keep the phone in your purse or elsewhere.
No. These are crackpot fearmongering claims. If cell phones caused cancer, the CIA and FBI would know it and their people wouldn't use them. The story about cancer being caused by brassieres was made up by a single anti-bra screwball in the 1990s. Don't give these people your attention.