The phone number feature allows you to enter a phone number or name and get phone book lookups in your Google search results. For example,

smith, boston ma
Finds people and businesses named Smith in Boston, Massachusetts.
starbucks, boston ma
Finds Starbucks in Boston.

Now, when you search the phone book like that you'll only get two results at the top of the page, with the rest of the results being Web listings.
You can search only the phonebook by using the "More phonebook listings" link, but why bother? Instead, you can go directly into a phonebook search on Google by using the phonebook: syntax.

For example:
phonebook:starbucks, boston

will get you a listing of the two dozen odd Starbucks in Boston, all on one page. Information listed includes business name, phone number, address, and links to two map generators for that location.

Let me give you an example of a phone book search that doesn't appear to work except when using the phonebook: syntax. If you search Google for -- smith, springfield -- you will not get any phone numbers. But if you search for -- phonebook:smith, springfield -- you will get two sets of results. The top set is business results: 5 out of 134. The bottom set is residential results: 5 out of 600. Phone numbers listed on the front page are from both Massachusetts and Missouri.

If that's still not enough for you, you can combine these with rphonebook: for residential listings and bphonebook: for business listings for extra fun (try: rphonebook: sears ma, bphonebook: sears ma)
I tried the following and I got the phone number of my brother in NJ!
rphonebook: Sameer Oak NJ

Best Luck with "phonebook:"