Clive Webster has written a tutorial over on expertreviews.co.uk which uses online service Pushover to send you an alert on your smartphone when your doorbell is pressed. He’s created the doorbell switch with a nice Adafruit button, hooked it up to the Pi and then played the sound of the doorbell from the Pi. It’s an interesting project. Be aware, though, that Pushover is a paid (albeit cheap) service that only works on iOS and Android. Read the tutorial here.