Remember times when you are getting calls from numbers you don't recognize.
There are people out there you don't want to talk to like your ex-boyfriend/ex-girlfriends, your creditors or simply those who just annoy you.
At the same time there are people you want and need to talk to, and you just can't afford to miss their call hoping they'll leave a message so you can get back to them.
So, ultimately you need to know who is calling you.
If the number happens to be residential land line or a business phone number; you might be lucky to trace it down using Google sometimes (not always).
You can also find some free directories online which list these numbers.
But these directories all not very comprehensive.
That means you will not find every number there.
Besides if it happens to be a cell phone number, you can not trace it through free directories or Google.
There is no official directory available for mobile phone numbers.
Cell phone numbers are not listed in free directories for privacy reasons.
On the other hand there are cell phone companies who keep all the data in their internal database.
There are a lot of companies out there who purchase data from these cell phone companies and compile all data into a huge database.
Since these companies are spending money to buy the data and also investing money to compile such huge databases; They charge people one-time fee to access this huge database for unlimited reverse phone searches.