Cell phone vibrator algorithm

So, sometimes my cell phone vibrator works and sometimes it doesn't.  I used to keep it on vibrate all the time because I've that that it ringing was annoying, but I have little choice when the vibrator doesn't work.

But I've figured out when it works with the following algorithm:

Was it recently dropped?  If not, it works if it worked before.
If so, its working randomly determined.

See, my phone has been dropped more times each week than an iPhone can and still survive, and so dropping it makes the vibrator work.