Skip calls with Remind Me Later
Alternatively, you can get iOS to remind you to call back later. As with the auto-replies, the way you do this depends on your version of iOS: in iOS 9 and iOS 10 you tap the Remind Me button above the slide, but in earlier versions you swiped upwards and selected Remind Me Later.
You can choose to be reminded in an hour, 'When I Leave' or (where applicable) 'When I Get Home'. Make sure your address details are up to date in Contacts, so your iPhone knows where home is. The timings will be based on your GPS movements.