What I have previously done (and replicated in my current setup which I have not yet tested) is to the use the "Callerid" field in Outbound Routes. Each of the iaxmodem entries has a "cidnumber" field (in /etc/iaxmodem/iaxmodem-cfg.ttyIAXn) which the install process sets to "00000000000", and "00000000001", and so on.
In you Outbound Routes field, create a dial pattern entry for each iaxmodem (the Incredible Fax creates 4 iaxmodems). Use something like 1NXXNXXXXXX for the match pattern, and then use the "00000000000", and "00000000001", and so on for the callerid in each one. Then all you have to do is direct it to a trunk that you only use for faxing (thus "dedicated").
(As installed via Incredible Fax that cidnumber is 10 zeros, plus the iaxmodem number 0-3, so the full callerid is 11 digits)
Hope that gets you going.
Jeff