Just like the definition of "cell phone" has changed over the last decade or so, the definition of the word "phone directory" has evolved as well.
Online reverse lookup services used to be simple, but these days the only thing that's simple is how to use them.
The only information they use to reveal was that which concerned the name, or possible the address associated with whatever phone number you were looking up.
Even then, it was difficult to do without already knowing the name, since old phone books aren't designed for reverse lookup purposes.
These days, in an age where it's sometimes difficult to have a lot of trust in anybody that's not family (and sometimes not even them), a lot of paid phone directories have stepped up to the plate, and begun to offer a much more comprehensive searching service.
Through the power of the internet and computer-based reverse lookup queries, it is now possible to look up all sorts of history on an individual solely through their cell phone information, including their criminal record, home address, and a wide variety of other information that used to be difficult to get hold of.
Today's online phone directories are smarter than they used to be, and they come equipped with a sort of intuition about giving you exactly the information that you looking for; no more, and no less.
In this way, they don't waste your time when it comes to doing research on phone records, while still giving you what you came there for in the first place.
On the other hand, and also much like today's cell phones, they tend to cost a bit more than traditional directory services.
Still, this is one of those situations where the means is justified by the end.