I have for years, occasionally heard a sound come through my headset, sort of a pulsing noise that goes on for a couple seconds every now and then. I've asked other people, including CFI's what those sounds were. Theories included, "no idea, but I've heard that once in a while, too", "uh...maybe static?", etc.
Finally, I was sitting in my office with my cell phone next to my office phone, and my office phone started making the same noise! This happened seconds before my cell phone rang. Aha!
I've never made a point to actually turn off my cell phone when flying, mainly because I once upon a time decided it might be a good safety backup for a dead radio or something. But after seeing the actual effect of the cell phone on nearby "instruments" (office phone, radio in plane), I now turn my cell phone off before getting in the plane. I can always turn it on pretty quickly if I really need it.
As for the laws, when I researched it a couple years ago, there was nothing in any FARs about it; it was only the FCC that was concerned with cell phone use in the air, although I've heard that the real problem was related to analog phones.