How do i quickly silence all notifications except for ringer on an iPhone 6?
Actually, Do Not Disturb is indeed your solution as it blocks ALL notifications EXCEPT the calls which you specifically allow. It will block message notifications. 

After you have configured the settings in Settings -> Do Not Disturb, all you need to do to enable it is swipe up from the bottom of the screen and tap the moon icon. 

You can configure it to only allow certain contact groups, favorites, all callers, and/or repeated calls.