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The National Cellular Directory is an organization which functions much like a land line phone company, keeping a database of individual cell phone numbers. However, it is an "opt-in" service, and to find a person there, that person must have voluntarily registered. People search organizations such as Intellius will quickly find just about anybody. A simple name/address/phone number search can be very inexpensivein some cases, under a dollar. If you do a lot of searches, though, you would probably want to consider a more robust plan. Most online searches will "find" a person for free. But to get any more information than that will have a significant cost. These organizations do not always operate with information that is up-to-date. For example, while they may be able to find the cell phone number of a particular person, that cell phone number may have been discontinued several months previously. Use caution when considering these organizations to ensure you are working with a legitimate company. Experience has also shown that those sites have incorrect ages or locations, even going so far as to offer to sell you the phone numbers of people who are long deceased. While virtually all the name search sites will charge you to view a phone number, there is a plethora of information out there on all of us, and our phone numbers may be attached to a number of things. School newspapers, social media sites, local organizations we may have joined, etc. Many times, people will make their phone numbers available to the general public. If you know the person and are trying to reconnect, friend them on Facebook, or make them a connection on LinkedIn.
Visit the National Cellular Directory. Pay for a service. Go to Google. Use Facebook or LinkedIn.