There are two parts to a service like this. The first part consists of the essential information you can deduce from any cell phone, even if you don't have a costly up-to-date database. This information is the city and state of your location.
If you perform a reverse phone number search, you will get all of this data and more. However, you will only receive this information for landlines. You can use it for a cellular number, but not vice versa.
Some of these websites also provide a wealth of other personal information on phone numbers, including marital status, criminal records, driving records, birth dates, addresses, and more. They also offer some demographic information, including sex, race, age, education, employment status, income, family members, etc.
As with any search, there are risks with a service like this. One of them is that you could lose a lot of money. The second risk is that you could get the wrong person's information.
Many of these free websites allow you to search by specific criteria. For instance, you can search by the person's name or address and the city and state in which they live. But, if you enter a false city or state, you may have no results at all. If you want to get this kind of information, you will have to pay a small fee.
Some free reverse lookup websites allow you to do a reverse search for as many times as you want but at a small price. Others allow you to do unlimited searches for one price. However, their databases are not as complete, so the chances are less likely to find the data you want when doing this type of search.