Keppler closes his clamshell phone twice after answering a call from Frank somewhere around the middle of the show.
When Catherine gets the phone call from Keppler calling her from Eric Hong's phone, the icons on Catherine's phone show it to be on silent mode, yet we still hear the sound of her phone ringing.
Shortly before the final confrontation, Willows receives a phone call in Suzy's apartment from Keppler. Immediately after, she makes a call to Grissom, at the end of which a man is visible in the darkened background on the right side of the screen.