"Dinner Reservation" is a codephrase for needing someone to dispose of dead bodies. So far, the only person shown to attend to "dinner reservations" is Charlie.
Also in the first movie, after John repels Ms. Perkins' attack on him at the New York Continental, he speaks on the phone with Charon. Charon says that he has heard numerous complaints about noise coming from John's room. John explains that he was dealing with an uninvited guest, referring to Perkins, who is injured and crawling out of the room. Charon asks if John needs to make a dinner reservation, a reference to needing someone to dispose of the body. John replies "Perhaps. I'll have to get back to you". Since John ends up sparing Perkins, he does not call back to make the dinner reservation.