It's easy to hit a brick wall when you're trying to do a free unlisted phone number search. Most online phone directories have records only for listed landlines, which isn't very helpful if the person has blocked his number or you're trying to call a cellphone. Luckily, there are free search engines that will dig deep into someone's personal information to find the number you're looking for.

Using Google's search field, type the person's name in quotes and include any other identifying information such as city or ZIP code. If the person has ever entered his unlisted number anywhere online, it may show up in the search results. Next, type the name into Google Images. Click on a matching photo and see if it takes you to a bio or social media account that has a phone number associated with it. Repeat the search at other search engines such as Bing and Yahoo to get the most well-rounded results.

Social networks like Facebook permit users to enter their phone numbers on their public profiles. If you're looking for a specific person's number, dig around his profile to see if it's there. LinkedIn is another great resource, although you'll need to create a profile before you can use its search facility. Once registered, search for the person by name to see whether he has linked a phone number to his account.

The National Cellular Directory promotes a one-hour daily "happy hour" during which you can access its premium cellphone number look-up service, 100 percent free. You'll need to create a free account to get in on the action. Once logged on, you'll see the happy hour status at the top of your page. You can also follow the directory on social media for updates about the next happy hour. To find unlisted cellphone numbers, type in the person's name. You get two free searches during each happy hour. If no results are found, the search will not count against your limit.

Pipl searches the invisible web to find unlisted phone numbers, which basically means that it looks in databases, registration-required websites and other "hidden" pages that Google doesn't index. There's no guarantee that it will turn up an unlisted number but, since it's free and takes just a few seconds to search someone, it's worth a shot. Just type in the person's name to get a phone number.

As with most things in life, you get what you pay for. While the above-listed sites will give you some information for free, you'll probably get better and more up-to-date search results with a paid service. Most companies let you pay for a one-time search or sign up for a monthly subscription to get unlimited searches. The unlisted phone number usually will be made available after you pay the fee.