It's a European company so they must be aware of the dreaded 'Data Protection' rules here. Users opting in to the service should be okay, but scanning and uploading their entire address books (the pre-selected default) with everyone else's contact info looks questionable. It's been done though, I know the Android Twitter app does this (it asks permission after installing and it's easy to opt-out) so there's probably a way of doing this without getting sued.

If you visit their site and try to search your number to opt-out, you get a modal asking you to sign in with a Google or Microsoft account - no other options are provided. In miniscule font, an even smaller tooltip says "Sharing is caring, and in the Truecaller community we care about accurate results. By sharing your contacts with Truecaller, you will receive more accurate results and help improve the experience for millions of users around the world."

It's a tried and tested business model but I'm just not interested in doing data entry for someone else's silo for a marginal benefit to myself. Maybe I'm old-school but I'm fine with figuring out who is calling from an unknown number by simply answering and speaking with the caller.