Boss: Please have Mr. Lee come see me.
Secretary: Sorry. but Mr. Lee has just gone out. ["Has just knocked off" is inappropriate for this situtation; it's much too informal. Note the office is formal enough to use last names. But even if it were on a first-name basis, "knocked off" would still be too informal].
Boss: Oh, then ring* him, if he's not at home yet tell him to come back to the office.
*"Ring someone" is rare in N. American English. "Call him". Or, better in context: "Get him on his cell phone".
Student or Learner