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Not OP, but I am lucky enough to have my own office with a door. Everyone in my group just leaves their door open if they are available to be interrupted. I communicate entirely by email and in person. If it's a quick thing that I only need a verbal response to, I'll usually walk over and see if the person is in their office (w/ door open) first. If not, or if I need something more substantial, I'll email.

On the flip side, most of the day my door is open, and I don't mind occasionally being interrupted to help someone else with whatever they need. If I have a conference call or I just want to really focus on something, I close my door and it's unlikely anyone will knock or otherwise disturb me unless the building is on fire.