This was always an undocumented feature, and it appears it is now officially gone and no longer works. People got disturbed when they found their phone number listed in Google search results, and publicly listed personal numbers are becoming the exception rather than the rule in today's world of mostly mobile numbers. Here's how Google's phonebook used to work. If you know the person personally, try emailing them or check to see if they've listed their number on Facebook or another social networking site.
Google's phonebook was hidden within Google. Occasionally, phone numbers would appear in the search results page, depending on the keywords you typed into the search box.
To access the phonebook directly, you used to type rphonebook: before your search for residential numbers and phonebook: for business numbers. It was specific Google syntax that's now been disabled.
For personal numbers, you generally needed at least a last name and a state. To find all the Smiths in Alaska, for example, you'd type phonebook: smith ak. That's a lot of Smiths, and probably not very useful to find a specific Smith. If you know more information, such as the city you're looking for or the full name, type that in too.
Google's phonebook could only find public phone numbers. It couldn't find cell numbers. Quite often the numbers are outdated. I found two outdated phone numbers for a relative of mine, and his current phone number wasn't listed at all, even though it is public.
Say you have a phone number and you want to find out who's number it is, such as from a message left on your cell phone. To do a reverse lookup, you used to type the phone number into the main Google search engine, including the area code. Type using the format 555-555-5555 for best results. Google will still find the phone number if you use parenthesis around the area code, but you may also find some irrelevant results. This still works, but it works by finding websites that list the results. You no longer get the neat in-line listing. It does provide great results for business numbers.
When Google does find the name and address matching a phone number for a business, it will also give you an handy link to Google Maps to get driving directions to that address.
This might look like a tool designed for stalkers, but Google was just putting together information already available from local phone directories and Internet phonebook searches. If you don't want Google to list your name and number, you could search for your own phone number and click on the "Request to have your name removed from this list" link in the search results. However, this was probably not enough to remove the public discomfort of being found in Google, so the search feature was removed.
Find Business Phone Numbers
Business phone numbers appear within Google search results, but they aren't as easy to access from the phonebook. The quickest way to find business phone numbers in Google is to search with Google Maps. If you search in the standard search engine, the search results will point to the maps.