Any Fax server has the ability to receive RBS T-1, PRI T-1, DID or Analog lines. The reason Analog was chosen in this case vs. the others, is because the fax card for an Analog Loop start config is cheaper, and provides inbound and outbound capabilities on all channels. Most PBX's (remember I'm a fax guy not the PBX guy) have the ability to pass the Inbound DID/DNIS information as DTMF tones down an analog Loop Start Tie line.
What I've seen from most of the other PBX venders out there is where they will configure the ports as Analog Voice mail ports/extensions, similar to setting up a Switch to work with a 3rd Party VM system.
Basically what happens, Call comes into the PBX from PRI/DID line, PBX takes the DNIS and translates to DTMF and passes an Analog Loop call to the Hunt Group that is pointing to the fax server. The call will ring into the fax server, and the fax server will go off hook receive the digits and then know where to route the call.
Does this make any sense?