How to block private calls on an iPhone?

I keep getting calls late at night from a private number on my iPhone. Whenever I answer, they hang up right away. I have no idea who it is, but I want to be able to block all private and unknown numbers. Is there a way to do this? I have AT&T and have an iPhone 5s with the latest OS. I know how to block non-private numbers, but I've been looking all through the settings and I can't seem to find any way to block private/unknown numbers.

Download a phone number blocking APP, then check off "block private numbers".

You can go with the above answer or more simply when you are ready to go to bed just put your phone on silent mode. If it is important enough they will leave a message.

If you are using AT&T carrier, The Blocking unwanted calls can be done by adding smart controls to your plan for $4.99 per month which allows to block up to 30 calls.

Follow the steps to subscribe.

Open I want to drop Down menu.
Choose change services.
Scroll down and find smart solutions.
Check the box next to smart limits for wireless parental Controls.
Click next.
Under features select the effective/Expiration date, to the choose the service Date when you want to start.
Hit Next.
Once you have added the smart controls to your plan, go to the smart limit for wireless on the account management page and choose the block numbers. Enter the number you want to block and hit submit.