Let the customer service representative turn the thing on or off depending on what they want
Don't try to modify the customer's speech in flight. Just recreate the speech entirely when it crosses a certain emotion threshold. "Now you better listen to me--" (Donald Duck voice kicks in). Guaranteed that dealing with angry customers would become a lot less unpleasant. You just gotta modulate, so that it's funny enough to preserve the call center person's mental health, but not so funny that they can't keep from laughing.