Tap and Hold the number you want to block from call History

Now you can see the drop down list.
Tap “block number…“

Read it through and Tap “accept“

Tap “ok”

Well done!
The number you’ve selected has been blocked successfully.
Let’s learn how to unblock the number as well.
Do followings.

Go to phone calls history again.

Tap menu icon  on top right

Tap “blocked calls” on the bottom

Tap “blocked numbers” box in the middle

Here you can see the list of numbers you have blocked.
Tap icon  on the bottom middle

Tap the box  to tick in front of the number

The box has been ticked 
Then Tap icon  on the bottom middle to confirm

Well done!
The number has been unblocked!