Block and Unblock numbers
BLOCK:Â From the Phone app, select the
Menu icon then select
Settings >Â
Blocked numbers.Â
Select ADD A NUMBER. Enter the desired number then select BLOCK.Â
Note: Alternately, from the recent calls tab, select the desired call > Block number > BLOCK. 
Call Protect
To automatically block calls from likely fraudsters, get alerts on suspected spam, and manually block calls learn more about
AT&T Call Protect. Free for eligible users.