Intertel has an interop document somewhere... give me a little bit and I will see if I can get it to you.
Hmmm... it is only for the Analog ones, but the same should still apply
Basically, make a Hunt Group and tag it as a VM hunt group and put all the FaxFinder ports in it (with SIP this may or may not require the VM unt Group flag, this MAY also require the Analog VM Hunt Group license, not sure), then make a system forward path that points to that hunt group, then make some phantom extensions and apply the system forward path you created above and select all calls always and from all destinations, then point the fax DID digits to the phantom extensions. the phantom extension numbers will appear to the faxserver and you make your routing in that based on those numbers.