TrueCaller built a billion-dollar caller ID data empire in India
When you install the app they steal your contact list and populate their database that way. Most people are unaware of this, or don't care since we are a third world country and privacy is seen as a first world problem. The vast majority of thinking here is not very sophisticated about these types of topics. Only a very tiny fraction of the total population is averse to sharing personal information like phone numbers. It's common to see folks post their tax ID numbers etc on public forums, tweets, etc. too.For a number of years it was quite common for folks in the lowest income brackets to change their phone numbers quite often because of rampant competition between mobile network providers. The "Mobile Number Portability" system was eventually introduced that minimized this to a large degree. Eventually the competition subsided and this reduced significantly to a point where it's not very common anymore per my understanding.When I need to use Trucaller I use it via their web interface exclusively with a google account that has 0 contacts for them to steal. I remember finding my number  listed many years ago as my name with (web developer) in parenthesis, likely stolen from some old customer of mine.