Unlisted phone numbers are those that are not listed on the white pages. In the USA, cellular phone numbers are 'unlisted' by default where as some land phone owners also get their numbers unlisted from the phone book for reasons of privacy. Wireless 411 Privacy Act, and many additional guidelines have been passed in USA for the protection of mobile subscribers. The reason triggering such measures is the persistent threat of such private information getting stolen and abused by numerous parties such as Nuisance Callers, Identity thieves etc.

This is why an unlisted phone number lookup requires you to use special online reverse cell phone number directory services rather than freely accessible white pages or yellow pages...