In my company (737 operator) we cycle the seatbelt signs to provide a chime in the cabin to let the crew we are about to take off. However we can only line up on the runway once we have had the secure from the senior cabin member.
Before the cabin secure is passed to the flightdeck, the crew at the rear of the aircraft call the crew at the front to let them know they are seated. This also makes a chime in the cabin.