The approach is free of charge, although it does cave some caveats beyond the initially limited device support. The implementation is still young, and many calls that haven't been labeled may still be authentic. You'll see that label more often as more carriers implement the underlying technology (SHAKEN/STIR). This doesn't guaranteed that someone calling from a real number is above-board, either. It could still be a robocaller, a scammer or a telemarketer.

This is a start, though, and AT&T is promising more phone support in 2020. Ultimately, this is more about discouraging call spam than anything. If all goes well, it could force robocallers to either use authentic numbers (and increase the chances of being blocked) or lose the interest of increasingly wary targets.