“Calm down” almost never calms anyone down. Try “I’m listening.”

Being told to calm down is heard as being told your reaction is the problem, which adds a second thing to be upset about. It also says, fairly clearly, that you want the feeling to stop rather than to understand it. Most of the heat in an argument is about not being taken seriously, and that phrase confirms it.

Say instead that you are listening, and then actually do it for a minute without fixing anything. If they are too far gone to talk, offer to come back in ten minutes rather than demanding a different mood. It works on you too, which is the quickest way to test it.