I need to implement the following scheme:
Is it possible on * (some calls are faxes but majority are voice)?
fax on G729 ?? what do you think compression is going to do to your fax data ? 
Nope, I just want codec to change to G.711 when * detects fax, otherwise go with G.729. However, as I found there is no way to do it without doing Answer() first which is not very good.
no, even conceptually you will need Answer() first. Unless some DIDs are dedicated to fax (thus can be routed accordingly), the only way to detect if a call is fax or voice is to listen to the audio coming in and see if it’s fax tones or not.
Also, how does one switch codecs from the dialplan?
What about T.38 then? Has anyone had any success with it?
there’s a thread 10 or so down !! forums.digium.com/viewtopic.php?t=8135