The answer is in the article. You "call" functions because they are stored in libraries, like books, and like books in libraries, when you want them, you identify which one you want by specifying its "call number".
Another answer also in the article is that you call them like you call a doctor to your home, so they do something for you, or how people are “on call”.
The “call number” in that story comes after the “call”. Not the other way around.