I bought a cheap door hanger that says "Meeting in Progress" and hook it on the outside knob. Used to just yell "I'm on a call!" but the sign works better. My host finally got the message to not knock.
My friend Jake had this happen last spring in a Portland Airbnb. He was on a Zoom with his team, using hand gestures while on mute, and his host came in to change a lightbulb. She later left a note asking if he needed a local therapist's number, which was kinda sweet but super awkward. He started leaving a sticky note on the door that just said "On a call!" during work hours. It feels weird to have to do that, but it sure beats those surprise visits.