This reminds me of my first job in London looking after the network of a recruitment agency. The consultants got headsets for the first time and one got so annoyed about being interrupted when on a call - because you couldn't tell when people were wearing headsets the whole time - that she taped a big bit of card to the top of her headset saying "ON A CALL" that she could flip up and down depending on whether she was speaking to someone.
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