My T works in a practice/clinic with a number of Ts. They all have signs on their doors saying (in approximate translation) "Conversation in progress", but the signs are always there even when the doors are open so they seem more like decoration than anything else. As far as I can understand they never knock on each others' doors or enter when the door is closed, though.

What I really like is the fact that there's a ceiling-to-floor portiere covering the entire wall where the door is, and which the T draws shut as soon as she has closed the door. That means that nobody could possibly barge in (they'd get tangled in the curtain!) and it also feels more secure and sound-proof.