Teens And Phone Conversations

Why is it that when you are on the phone in a detailed conversation, your teen thinks it's okay to come right in and talk to you like you aren't busy, but the minute that you knock on their door, go in and try to ask them a question, they signal to you that THEY are on the phone and want you to be quiet? What's wrong with that picture? Although my teen is very respectful, she still seems to exhibit this particular habit. I don't know why, it's not like it's any secret that she has a mother that needs to ask a question, now is it? Teens, I just don't know where they come from sometimes.