I've been using nomorobo (app for $2 a month) that uses a massive blacklist to block robocalls. It works really well and you can share whatever calls sneak through to them to add to their blacklist.

It's better than the do not disturb option because it doesn't also block text notifications.

Really this would all be massively simpler if iOS allowed your contacts to act as a whitelist and just push all other callers straight to voicemail.