Know who called you:
TrueCaller maintains a huge database of contacts and help your display name and phone number of people who called you, even if that contact is not in your phone book.
Truecaller integrates with phone-call to show name and number of person if its unknown and helps you decide to receive the call or reject it.
In case you get a miss-call from unknown number, TrueCaller show you name/number of this caller. You don't need to return a call or text back to know who the person is.
How does TrueCaller gets this data:
As per TrueCaller, they get it from public sources, yellow pages, people who wish to submit info about people from whom they got calls. TrueCaller do NOT use your phone book to send info to them for their usage.
When you register to TrueCaller, your number gets added to their database. People can know that you call calling, if they too have TrueCaller installed. You too get advantage to let people know that your are calling them even if people don't have your contact details in their phone book.
Block unwanted spam calls & Text :
Marketing and sales calls are common. You may register to 'Do not call me' and avoid people calling you in first place, however its not standardise and globally available. You may still get spam calls. Best way to avoid spam calls is to use TrueCaller. It guards you with regional spam list and blocks all calls/message.
You can add custom list of contacts that you wish to block for calls/text.
TrueCaller database for spam numbers will continue to grow as users around the globe submit data, you will get the benefit of it and all unwanted calls get blocked automatically.
Privacy on Truecaller:
Your are exposing your name and number on TrueCaller. They get a valid phone number which is maintained in their database. Even if you don't install, your contact details may get added by your friends or third-party as they interact with you. You can always choose to remove your contact details from TrueCaller website by visiting here -
You are installing an app that gives away your name/phone-number to public website. You need to configure in such a way that only your friends should be able to search you and not general public. Enable below settings in TrueCaller after you install it.
TrueCaller request for login using facebook/google credentials. I would recommend NOT to use facebook/googe+ account as it post status on behalf of you on your facebook wall. Also it can get access to all your Facebook data (friends/location/email) which is too much of ask by TrueCaller app.
How to get TrueCaller:
Download TrueCaller on Android/iPhone/Windows phone http://www.truecaller.com/