Knock Knock

One of my duties at the store is to check the four fitting rooms, every half hour. In doing this, I become aware of society's lack of an established greeting for fitting room interruptions. After knocking on the door, I get everything from "Hello?" (are we on the phone?) to "I'm in here!" (don't worry, help is on the way) to "Who's there?" (to which I am strongly tempted to reply, "Interrupting cow"). Somebody needs to figure this out.