The CubiCaller is a way for folk to announce their arrival at your workstation without resorting to clearing their throat, or worse, saying "Knock knock!" in an overly jovial manner.
A battery-powered device, it attaches to the walls of your tiny, tiny world with a self-adhesive backing.
Depending on how noisy your office is the three position volume selector lets you set the loudness for your particular environment.
There are 12 differing ring tones to chose from including chirping birds, a car horn, chimes, a fog horn, a duck quacking, a cat miaowing and just a plain ole fashioned "ding-dong" doorbell.
$11.99, link below.