The fox is not at all concerned with Hiei knowing what he is up to at the moment, anyway. If he had his own phone with him, this wouldn't even be an issue he had to be pulled into, and Kurama could still be in the room reading his book. Unfortunately for everyone involved, Hiei could give them both the cold shoulder as long as he liked, and that wasn't going to change anything on their end, not when they both continued to prove more stubborn than him in many regards.

“On the contrary,” While he catches the amusement Kurama is not reading the tone as indication of a joke, rather that the fire apparition finds the suggestion absurd. As he should, because it wasn't the fox's intended meaning. “I believe you should probably avoid that. After all, who knows what might happen, in your current state.”

Of course, that same logic doesn't apply to the redhead, who could probably really use a boost to the effects, himself. Not that he's going to admit to as much out loud.