Every time I have tried to find a free site that does this, they lure you in with a free lookup. They find the person you are looking for, give you the possible related people and when you click on the tab that should give you the information you are looking for, it takes you to the page where you choose the package that you have to buy to get the informaiton you seek. Only the initial search to find the person is free. The information will cost you.
If it is a landline, you can you Anywho.com, which is an online white/yellow pages site.
The bottom line is that if the online sites other than Anywho, will cost you.
Intellius will cost you, but they seem to be pretty good. However, all of these sites may not have the most up to date information. They have been linking me with my ex for the past 15 years.