I've been calling a mentally ill person to hear him gasp.
Why do you do that to him?
Well, I'll tell you one thing. He's kinda disturbing.
Intentionally antagonizing someone who is mentally unwell is extremely cruel. You are causing them harm for no reason other than for your own amusement. You need to stop doing this and be kind toward him.
I did it because I felt good watching him gasp.
I don't quite understand why this is enticing to you, but I have to emphasize that this is not harmless to the other person.  You could be causing real emotional distress to this person, because he has no idea who you are or why you are trying to contact him.  I suggest finding other ways to amuse yourself.