The Crazy Young Master's Daily Life Is a Scheme

Woojung was enjoying a peaceful life at work until a troublesome man named Jooshin suddenly appeared! As an intern, Jooshin causes trouble at every turn and constantly picks fights with Woojung over the smallest things... As his mentor, Woojung tries her best to be nice in order to turn him into a competent employee who can actually pull his own weight. But then...

"Manager, why did you change your number?"

What does it mean?

(Source: Naver, translated)