I am assuming that your faxes on on your IPT netrwork. What protocol do you use to control them (MGCP, H323)?
If MGCP then you simply make sure that the CSS assigned to the FXS port has access to other FXS port paritions.
If it's H.323 then you need to configure dial-peers pointing to other fax machines and make the session-target ipv:, also make sure you use G.711 codec and wheater you want to use pass-through or relay by adding appropriate commands. Make sure the H.323 GW is configured on CCM.
HTH, please rate all posts!
Chris