IDIOMS

wrong number

(idiomatic, dated) An incorrect notion or understanding concerning a person or situation.

Example: A telephone call received from an unfamiliar caller, due to a mistake in the number dialled.
1920, Ring Lardner, "The Big Town":
  Codd said . . . when he had her fixed and tested her a few times he would take me up for a ride.
  "You got the wrong number," I says. "I don't feel flighty."
1984, David Homel (translator), Jacques Renaud (author), Broke City (original French edition, Le cassé, 1964), ISBN 9780919349377, p. 37 (Google preview):
  The readers probably expecting me to go on about how Johnny was real nostalgic about some kind of material security. . . . Well, the readers got the wrong number.
2005 Feb. 20, Richard Taruskin, "Music: Restoring Comrade Roslavets," New York Times (retrieved 1 June 2014):
  His musical system, like the political system he served, would rescue the present moment from crisis and place it at last in productive harmony with history's demands. Boy, did he have the wrong number.