If you want more details you can go to:
The basic idea is actually very simple, we test rigorously and then we test some more. We found 45 different reverse phone directories to test. We then used ALL of them ourselves, running approximately 170 commercial, residential, cell, and unlisted phone numbers on EACH service.
This took us six weeks to complete and we ran more than 8000 numbers in total. We had to sign up to both premium and “paying for a single number lookup” services to complete our tests. We spent more than $1000 paying for this. (There are certain aspects of our tests that we have to keep a secret due to the fact the phone directories might change their services just to pass our test.)