Unlisted numbers are the numbers that are not found in any public directories. If you search for those numbers in any public directory, you will get a message like, "the number you are searching is not available in our database", or "number not available", or "cell phone number", or "private number." It is almost impossible to identify the owner of such numbers unless you get some professional help.
The owners of the unlisted phone numbers usually don’t want to list their phone numbers in public directories because such numbers are mostly used only for making calls, not receiving calls. One good example of such numbers would be the number of the telemarketers. Telemarketers call random people to promote and sell their products but they don't want their numbers to be listed anywhere to be seen by public. Beside telemarketers, other agents like debt collectors, anonymous pranksters, and survey companies also usually delist their number from public directories. If you want to know who have called by using their numbers, it is highly unlikely that you will get any result on any public phone directories like 411 or white pages or yellow pages for such unlisted numbers.
One approach to identify the owner of such number is to search using public complaint boards like 800Notes.com or Whocallme.com. These websites are repositories of public forum posts on many numbers that periodically call us.
Another most convenient approach to find about such number is getting a professional assistance. Using paid services provided by websites especially dedicated to gather data for private numbers using multiple sources is the best way. One can easily get detailed information on any private number easily by searching online on such database after paying some nominal fee.