There are settings that allow calls to bypass Do Not Disturb. On the Mac in System Preferences > Notifications > Do Not. Disturb there is an option to Allow calls from everyone and another to Allow repeated calls (a second call from the same person within three minutes will not be cancelled). On your iPhone in Settings > Do not disturb there is an option to allow calls from Everyone, No one, Favorites, or specific Contact groups as well as repeated calls. Have you checked to see if any one those Do not call exceptions are enabled?
