Most systems that can accept a PRI can handle sending what the fax server needs. The scenario is, a fax call comes in on a certain DID which is programmed as one of a group of "fax DID's".
The system routes the call to the specified extension (a single extension for all "fax did's or group of extensions if the fax server has multiple ports).
When the call reaches the fax server, the phone system sends DTMF digits that show the DID number called. (similar to how voicemail systems know which mailbox a call is for)
The fax server receives the fax and routes the document to the intended user(s) by the DTMF digits it received.