[b]Not him, he just opens my door, walks in and starts talking even if I am on the phone. I try to ignore him but he just gets louder. It gets to the point where I have to say. "Excuse me" to whoever I am on the phone with and then look at him and say, "I'm sorry I am on the phone" at which point he still insists on telling me what he wants[/b]
Time for a three-fold solution:
Put a "DO NOT DISTURB" sign either at eye level on your door above the doorknob or taped to the doorknob itself. Put up the sign (obviously) only when you go to shut your door for a call.
Should he barge in anyway, this is when you aggressively stick out your arm with your hand out (stiff armed), palm facing towards him. Should he continue talking, don't reward him by stopping your conversation. Instead, hunker down over your phone and stiff-arm shake the palm of your hand at him. If he continues, stand up and wave him out of the door (with your same hand) with a pained expression on your face, closing it promptly behind him.
Afterwards, go to him and say, "When I have my door closed, that means I'm on a call that can't be interrupted. That's the only time I have my door closed. And, you'll also see my "Do Not Disturb" sign."
Also, you CAN go to see the owner even though this guy is his friend. You just have to approach it as though you are seeking his advice as opposed to making a complaint. Something like, "It seems that Greg thinks I am an admin because he continuously asks me to post his letters, check his voice mail, FedEx packages for him, etc. And, when I'm on calls with my door shut, he talks over me and I've been having to put the client/vendor/whomever on hold. How would you suggest that I handle this?"