Basically, I want to do the same thing that Google Voice does. I forward my calls to a DID number that rings my Asterisk server via IAX2, which, if it detects the call has been forwarded, sends it to voicemail. Otherwise, if the call hasn't been forwarded (and somebody has dialed the DID number), it dials my phone number and tries to reach me. tl;dr: how do I detect that a call has been forwarded to my asterisk box?