Thanks tdol for your comment. :)

But as I pointed out in my original question, the very reason I am having this question is that there are many wrong numbers you can dial too.
If you are talking about one specific instance among many other possibilities in both cases, why do you usually use "a" in "a nice haircut" and "the" in "the wrong number"?
I feel there might be a logical difference between the two situations that results in the two different choices of the article (a/the) but I don't know what that is.
Or is it just a grammatical quirk that has resulted in the regular use of "the" in "the wrong something," making it an idiomatic expression?

Is my question making sense to you? :?