Taebun: What do you think when I say "Answer the call of nature?"
Me: I think you need to go to the bathroom. You can also say "Nature calls!" and leave the room.
Taebun: What about when I say "Number 2?"
Me: You need to poo.
Taebun: Is it common to say?
Me: Haha, yes.
Taebun: You could say it to your friend?
Me: Yes, if you want to.
Taebun: Could you say it to your grandmother?
Me: If I want my grandmother to know what I'm doing in the bathroom, then sure.
Mrs. Im: How about "number 1?"