I've tried this on several numbers even Unknown & Private and it has worked for me most of the time.
For those with home line AT & T: To Block a Number From Calling You
1
Press *60 on your phone and listen carefully to the announcement. It will tell you whether your service is on or off.
2
Press 3 to turn this service on (or off), then press 1 to hear the numbers that are on your blocked list, if any.
3
Press #01# to add the last number that called you to the list.
4
Press # to add a new number to your list. The voice instructions will tell you to enter the area code and number, and then press # again.
5
Press * to remove a number from your list. Voice instructions will then tell you to enter the area code and number, and then press #.
6
Press 0 to hear all of the voice instructions again.
7
Deactivate selective number rejection by following Steps 1 and 2.
Read more : http://www.ehow.com/how_5025922_use-call-blocking-at_t.html
I love my new home/landline phone; I can block these unwanted calls who never leave a message. Why would someone make a call and hang up. This used to happen when kids got out of school for the summer and got bored.
Telemarketer was selling air conditioner service. I advised him of the illegal call and I hung up while he prattled on. The return number is listed as not in service - surely a sign of a criminal enterprise.