Sirius called Kreacher, and Harry called Dobby. The task was explained to the house-elves: "We have something that we won’t order you to do, but we ask you to do it." Snip. "After you levitate the horcrux into the lead box, elf-pop to Gringotts and give the box directly to Director Ragnok. If Ragnok tells you to wait for a message from him to us, then wait."