Ask Apple: Why there's no easy way to block unsolicited SMS/calls on iPhone?

Try using the Do Not Disturb feature in combination with your Contacts database. All calls from numbers not in your Contacts are ignored, which is almost as good as blocking them outright. Also, these callers are not inclined to leave a voicemail.

They just released https://developer.apple.com/reference/callkit for iOS10

If you're on iOS 10 go Settings > Phone > Call blocking and identification.