So one day the father invites his subordinate home. The son looks through his cell phone when he's not looking. Since that day, that boy sends him texts pretending to be a girl. One day the subordinate wants them meet up, and the boy gets frantic as he doesn't know what to do. But the subordinate actually knew everything all along. They end up together in the end.

I think it's from a collection of one shots, and this story is not the first chapter,, i think,,
Anyone know what manga is that?

Thank you very much ^_^

Might be from Syrup! - Bitter, but unfortunately not sure which oneshot specifically.

Hey, you're right! it's from Syrup! I checked it, ch 14 Konna Kanji de Hajimarimasu.
Thank you so much, tokkei12~ ^,^