Call bid adjustment has been available for Search Network since July 2017 on campaign level. You could find it in Advanced bid adjustments.
Now, Brent Neale and Adriaan Dekker shared that it has moved to the Call asset configuration (available under Assets = Ad extensions).
Google probably wants to streamline the interface, as the new location makes more sense than its previous location, where it was often overlooked.
Here’s how the call asset configuration looked bafore this change:
