If you want to search for any contact from Truecallers web interface you’ll need to sign in using either Facebook, Yahoo, Google or Microsoft accounts.
Actually it is using all the data uploaded from all around the world by the people like you and me to show information of some specific number. When someone searches for the mobile number or a specific name, Truecaller uses numbers and their information uploaded from your phone to show results for that search.
When you install Truecaller application on your phone, it simply uploads each and every contact and information linked to it from your phone’s contact list to its servers. You might wanna know how Truecaller actually works? Truecaller gets permissions to track your precise location so that people near you can search and access your details much more easily. Don’t you think this is not fair with others? You are giving away their information without even asking them. By using the app you are giving away your and everyone else’s information – stored on your device – to the company, which they use to show names and addresses to others who search for some phone number or name. They are getting that information from all of you, users who are using the Truecaller service. But here is a thing, did you thought from where Truecaller is getting all that information? Truecaller is a great application/service, it can tell name and any other available information of the phone number even if you don’t have that saved in your contact list.