An unlisted or unpublished phone number is a telephone number whose owner's details can not be found on public directories such as the white pages and the yellow pages.
Every individual has the right to make their telephone number unlisted or unpublished if they want.
Due to privacy laws, the telecommunication companies have no right to make available the details of the owners of unlisted phone numbers on the public directories therefore you can not conduct a free unlisted phone number reverse lookup search on directories such as the white pages and the yellow pages.
Tracing an unlisted phone number back the owner of such a telephone number can be difficult as the details of the owners of such numbers are not always made available on the public directories due to privacy reasons.
However, there are business or websites on the internet that work round the clock to gather information of the owners of any types of telephone numbers, compile these information into their simple to use database which they call a reverse phone lookup directory.
These companies are not only able to help anyone find the details of the owners of an unlisted phone number but they provide services for mobile and land line numbers as well.
They can disperse this information at a certain fee that is reasonable and affordable for virtually everyone.
The paid reverse unlisted phone number lookup charge a fee in order to recoup tend to hold on to their integrity and they mind who they give access to the unlisted telephone numbers because they know if the numbers get into the wrong hands, then a lot of damage could be done to the personality that is behind the unlisted telephone number.
So if you are looking to do a free unlisted telephone number search, then you might just be looking for information that may or may not be reliable.
I believe putting in a little fee will not be an unworthy venture so far the reason you are looking for the unlisted phone number is justifiable.