yea, basically lets say your DIDs are 4100 through 4199. You need to set those DID's up as voice hunt groups. Then call reroute those hunts groups to a hunt group that you created that have your ons ports in it.
To test it, plug an analog phone into one of your ons ports. Then make a call to the DID. When the call rings the analog set, pick up and you should hear it dial the DID (4digit) that was dialed. If you hear that, you should be good to go to plug those ONS lines right into the fax server.