Jessica, Trish and Simpson made a plan how to catch Kilgrave:
They planned to put him in the chamber which is completely isolated. Even sound can't pass through it. However, later she talks happily to him via a microphone. They do this because his powers do not work if he does not directly speak to you. In a way he is like a virus.
But how could they have known this would work before trying it? Sure, she talked to him via a cellphone before, but it could have been that he just needed to be nearby for his powers to work.
How did they know beforehand about this fact?