You've got the wrong number.

I am talking about phone calls. Do you never say "You've got a wrong number" ? I don not quite understand why you say "You've got the wrong number". I hope someone can help me to understand why "the" is used and not "a".

Likewise,in the following dialogue, "the wrong message" is used. To me, "a wrong message" seems natural. Someone please also explain why "the" is used. Does the matter have something with the word "wrong"?

A: I’m concerned about the eating habits of Leo may be developing.

B: Leo is a picky eater. He’s always been that way.

A: Nonetheless, he shouldn’t be eating cookies and marshmallows for lunch.

B: Pardon me?

A: That was his lunch --- cookies and marshmallows!

B: I had no idea!

A: Packing a sugar-filled lunch sends the wrong message to children.

B: I agree.