http://www.trapcall.com/
Pretty neat loophole they're exploiting. And the basic service is free. Now if they could only expand to Verizon.
"It's actually a fairly simple procedure. TrapCall reroutes unanswered blocked calls through a 1-800 number then back to your voicemail. Since anonymity is denied to 1-800 numbers, TrapCall simply picks up the number as the data moves through their servers. According to a Wired test, the rerouting process only took about 6 seconds on AT&T—and during that time all the caller hears is standard ringing."