Cold calls from telemarketers and other companies that ignore the do-not-call list are one of the banes of modern day civilization.
A simple Google search for a mystery number you've received usually lets you know who's on the other end before you have to pick it up. The problem is that cell phones don't have the same quality of caller ID landlines get (numbers not names); so that call you're getting could be something important like an overdue library book, or a pushy desk jockey trying to sell you a heavily discounted hafnium-forged non-stick pan set.
In most cases the telemarketers don't leave messages and will simply call you back, resulting in an endless cycle of you not knowing who's calling and having to call back to find out--something you're unlikely to do. To avoid this, there's Caller Complaints, a crowd-sourced index of the phone numbers of law breaking companies that have called folks on the do-not-call list. Users come together to list these numbers, what was being pitched--and the frequency of the calls. If you find someone else has already listed the number and shared their negative experience, you can pile on and leave your experience, which votes it up.
The most popular (or in this case unpopular) companies rise to the top and are tracked on leaderboards. Users can also browse by area code and what type of call it was, from political phone spam to prank calls and debt collectors. The idea is that there will be enough resources to help you get to the bottom of who's calling to either leave a complaint with your carrier or simply blacklist the number from calling again.
So far the site has amassed nearly 200,000 number searches from curious call recipients. If you're adding a number to the database you also have the option to do a little quick research on ReversePhoneDetective, which will tell you where the call originated from and give you the option to pay for a full report.