Requires Trucaller app be installed on the device, beyond this, no information on what activity or information the app itself gathers, only mentions they don't track your app's activity, which is meaningless in context.
No clear information on the Trucaller revenue model at all, and unclear on any other potential gotchas.
You're probably better off using Twillio or another SMS backend with OTP verification methods in practice. Aside: I just don't trust outsourcing your authentication, I think it's a bad idea to literally outsource your user access and has so much potential for abuse. Similarly I wouldn't recommend using Firebase or Auth0 as primary authentication interfaces either.