Continually trying to get a signal a 30,000 feet through the metal skin of the aircraft tends to do that to a cell phone battery.

FYI, the proposals to put cell service on board would provide a mini-cell tower on the plane, which in turn would cause cell phones to use a lot less power.

For anyone who's interested in the whole "are cell phones dangerous onboard" issue, there are plenty of really long FT threads on the subject. And very little agreement.

But the question in this thread is what to do if someone decides to use their phone in flight. Generally, I point out to them that they're not allowed to use it (as though they just weren't aware -- yeah, right ) and that gets most people to stop. If that didn't work, I'd certainly bring it to the attention of the FA's.