Template:Software Wishlist This is an application, which listens for the user whistling a tune, and then does some action (ringing, ...).
The CPU is put into a low power mode where it still has enough compute power to take the signal from the microphone, divide it into many frequency bands, then store these in a short history.
Every second or so, it looks over this using a fuzzy matching algorithm for a signal matching a whistled (sung, yodelled, drumbeat, ...) tune, and takes an action when the probability that it's heard something is over some figure.
This allows the user some flexibility in both pitch and timing, with more notes being required in noisy environments, or when the user is far from the phone.
As an option, the phone might listen in this manner for only a minute every quarter of an hour.