Who is it?

Sometimes I pump in my office. The door doesn't have a lock so I put out a sign that says "Conference call" and jam a chair in front of the door. I forgot the sign the other day when a co-worker knocked on my door. I almost yelled out, "Who is it?" but realized that no matter the reply I could not act upon it. So, I sat very still and as quietly as the pump would allow until they went away. :)