I bet I'm not the only one who sometimes puts headphones on with no music, solely to broadcast that don't-bother-me signal.
Current workplace has a no-headphones rule, exactly because of that.
Many people were donning headphones as soon as they arrived in the office and not taking them off until home time, simply to avoid having to interact with anyone else.
So now no-one can avail of shorter periods of non-interruption, but I understand why they banned them.
In-ear earphones seem to be tolerated, so long as they are removed when someone comes to talk.
Either way its something to tap to find out what's going on.
Instead I use earplugs.
edit: unlike any other earbuds I have tried, they are engineered to survive the inevitable wax-plugging. Just replace the filter & you're back in business.