Q for Japanese. When you are in a toilet stall and someone knocks on the door, what are you supposed to do?

Question:

The other day I was dropping a deuce in the TOTO toilet at the school I teach at and some dude knocked on the stall door to see if anyone was in there. I read somewhere you are supposed to knock back, but that just seems strange. So I just said はい and the guy went away. So is knocking back the appropriate thing to do when someone is knocking on the bathroom stall or are you supposed to say something? A million thanks if you could answer this.

Answer:

You can just knock back, twice.... knock, knock.
You could also say, "Haitte masu, 入ってます"