iPhone
Go to Phone, then ‘Recents’, click on “I’ alongside the number you wish to block, then scroll down and select “Block this Caller”.

Android phone
Select the number you want to block from the recent call log, hit the three-dot menu in the upper right of the screen and choose “Add to reject list”.

Windows Phone
Open the call list, press and hold the number you want to block for a few seconds. A small menu will appear, select the “Block number” option.