I had an incident at the Westin Mich Ave in Chicago where the housekeeper kept ignoring my DND on checkout day and harassing me to leave (I had 4pm checkout, but even after I notified her of that, she persisted).

Just this past weekend, while at the Sheraton Park Anaheim, I witnessed the phone trick as I was approaching my sister's room (family trip to Disneyland). The housekeeper knocked (ignoring the DND sign). When no reply, she went into the adjacent room and called and said, "You're DND sign is up. When will you be out of your room before 5 so we can make it up?" Kind of defeats the purpose of the sign...