On the ball means alert, quick and on top of everything. Really on the ball, not on the ball today, somebody on the ball.
It praises attention rather than cleverness, which is why it suits receptionists and referees alike. The picture is from sport, where keeping your eye on the ball is the whole job.
1phraseon the ballAlert, quick, and on top of everything.
“The new clerk is thoroughly on the ball.”
on the ball is a phrase.
Practised in Idioms