I stumbled across this little nugget in the Asterisk docs, and I thought other people might find it usefull as well.
I remember buying one of these for my dad for his POTS line a while back (heh, 10 years) , from Radioshack of course. The idea is simple, a tone played down the line makes the autodialers believe your line has been disconnected.
This functionality has been built into Asterisk, here is the dial plan code:
[incomimg] exten => s,1,Zapateller(nocallerid) exten => s,n,Playback(enter-ext-of-person)