Ding dong bell in flight?

Every time I'm on a plane somewhere I always hear a ding dong chime in various phases of the flight. It's always right before takeoff and at completely random times during cruise. I can understand that it's used to alert the flight attendants that takeoff is imminent, but what is it's purpose during cruise? And can someone explain how the flight crew makes the chime (I heard you turn the seatbelt sign on and off twice but that sounds kinda weird)