Murphy's Law of Mobile Phones

The phone sits there, quietly biding its time, resting.  Then, when you pick it up and actually need to make a call with it, it tells you the battery is low and needs charging.

There might be a corollary which explains why the standard, low-battery signal is a quiet chirp, but if you're in the middle of a call, it is an extremely loud BLEEP-BLEEP-BLEEP-BLEEP which not only obscures what the person on the other end of the line is saying but makes you temporarily deaf as well.