Create a chart the student can use to self monitor. This could be a small card ruled into boxes or a card with the different lessons listed for the day.
Each time the student calls out they put a cross in the corresponding box eg if it is the first lesson of the day they use the first box. If they do not call out during the lesson they tick in the corresponding box at the end of the lesson.
At the end of the day encourage the student(s) to look at whether they reduced the number of times they called out during the day. Keep charts from previous days and encourage the student to look at whether they have improved from previous days.
If at the end of the day they have a tick in each box they could earn an agreed on reward. You could also choose to set it so the students receives an agreed reward at the end of the week if their number of ticks has increased. Ensure the reward is something the student wants to work towards and something you are able to give.