Not to be Captain Obvious or anything, but the book opens with a monster coming to call on Conor. Boom. Title explained.
Ah, but not so fast. Another monster's been calling all along, if only through his subconscious. And that monster is death—the pit monster. So we've got the relatively friendly yew tree monster, and the creepy, scary pit monster, duking it out for the honor of who's named in the title.
None of the characters in the book ever speak the title, nor does the narrator, but we know that both these two monsters call before the book ends. After all, good monsters and bad monsters call on all of us. And it's up to us to figure out whether or not to answer.