Filter Calls for Blackjack?

i keep getting prank calls that say the # is witheld and i would like to filter my phone so that my phone only rings when someone in my contacts calls so the prank callers only hear a busy signal. i dont want to pay for anything extra just wondering if this is a setting i have on my phone i also dont want to call my phone company (at&t) because they will just make me more confused thx for any feedback you can give me. btw i hve the first blackjack from samsung NOT the new one.