"Hello? Hello? Anybody home?" was an insult used by Biff Tannen.
Whenever he demanded "thinking", Biff knocked a victim — such as George McFly in 1955 and 1985, for example — on the head with his closed fist as if he were rapping on a door, saying loudly "Hello? Hello? Anybody home?!"
Biff was still using this insult as an old man in 2015, but because his hands were by this time too feeble to hit anyone hard, he used instead a cane with a metal fist as a handle to carry out the knocking action — the victim on this occasion being Marty McFly, who was posing as his future son, Marty Jr.