If a number is private, you cannot find it in a directory. If someone claims they can sell you that information, it is likely a scam. The telefonski imenik po broju telefona is no longer a single book you buy at the newsstand. It is a combination of Google, Facebook, and crowdsourced apps like Truecaller.
Here is everything you need to know about how reverse search works in our region. A standard phone book works on the principle: Name -> Number . telefonski imenik po broju telefona
This means that will give you the name and address of a private person just by typing their mobile number. If a number is private, you cannot find it in a directory