Google Duplex is a feature that lets the Google Assistant make reservations on your behalf. The company is starting to roll out the feature to iPhones and other non-Pixel phones.
The support site for Duplex lists the iPhone as being part of this rollout, with Android phones simply requiring Android 5.0 or higher to use the automated calling service. iOS users simply need to have the Google Assistant installed to get started.
Check It Out: Google Duplex Rolling Out to Google Assistant on iOS