asterisk users - Jan 2005 - FAX detection in extentions.conf

Hello

I have a PBX for our company with in front a ISDN * ISDN server.
I would like to detect if the outgoing  line is a fax call or  voice call.
I know ther's a fax detection in zaptel.

Now the question can I make desision in extentions.conf when * detects a 
fax call.

FAX > PBX > ZAP/G1 > ZAP/G2  >  the world
and not
FAX > PBX > ZAP/G1 > IAX2  > voip world

Thanks