Google knows everything. Apparently, it knows about most of the phone numbers as well. Most of the times, whenever you get a phone call from an unknown telephone number, you can simply search for that number on Google and find more information on that number. If there are other people who are receiving similar calls from the same number, they probably have posted about their findings on some blogs or websites.
Google actually is a very good resource if you want to perform a free reverse phone lookup. With my personal experiences, I have found that Google can provide you more information about majority of the phone numbers. One time, I attended a seminar on web marketing. On that seminar we had to fill in our information on a sign-in sheet. We were asked to enter our phone numbers there. As per their request, I did enter my actual phone number. After a few days, they published the entire list of attendees with their respective phone numbers on their website. Even though it was not a very good thing to do if you consider people's privacy, there still are many companies who do it anyway. I sent them a request to take down my information from their website, and they did. But, other attendees who haven't sent them requests still have their names and phone numbers listed there. So, if someone from that list calls some random person, and that random person goes on Google to look up that number, they will be able to identify who the caller is.
To verify if Google is a good resource for reverse phone lookup, I recommend searching your own number on Google. You may be surprised by the number of results you get from your search. Some of those results may even contain your personal information. If you are able to find your own information on Google based on your telephone number, you can be quite certain that it will help you find information on other phone number owners as well.