If you're seeking an old friend but only have an out-of-date phone number, don't worry. There are plenty of very efficient online resources that can help you find the person you're looking for. It might take two or three online searches, plus some legwork on your part, but it’s perfectly possible to find someone with only an old number. Plus, most of the resources are completely free to use.

A simple Google search can turn up a lot of information, so start your search at this popular search engine. Enter the number in 10-digit format, for example, 555-666-7777, then press enter and review the results. The number might be associated with a business or organization, people’s names, posts on social networking sites and blogs; all of these are viable contacts for you to investigate. For example, if the old number is listed on someone’s Facebook page, send a message to that person, explain who you are and ask if they know the person’s current contact information.

Conduct a search in an online database such as Zabasearch or Pipl. All you have to do is enter the phone number and see what information comes up. Often, a reverse phone number search can turn up a person's name and current address. The algorithm might also throw out some other useful contact information such as a current phone number and the details of family members, who you can possibly trace and call to find your friend. Sometimes, you'll have to pay a small fee to get a person's current contact details.

You can also enter the number in Phone Validator which displays the location for that telephone carrier. Dial a 411 directory assistance number, such as 1-800-FREE411, ask for listings for the person’s surname in that city. Then, call those numbers. You may find a relative who can give you current contact information.

It sounds obvious, but calling the number may help you get updated information. If the number is a landline, for example, and is out of date because the person moved, then the new homeowner or tenant might have some information about your friend's whereabouts.

Hire a private investigator. If you hit a dead end, it might be time to pay for the services of a professional. To find a qualified PI, contact your state professional private investigator association, ask friends for recommendations or contact attorneys and request recommendations for PIs who specialize in locating people.