Asterisk, only having T.38 pass-through available means you will need to connect the HylaFAX Enterprise server to either a T.38 gateway (Digium and Sangoma boards are not gateway devices and unfortunately do not offer T.38 functionality) or make calls from HylaFAX Enterprise to the PBX with G.711. If using G.711, you will need to have both the HylaFAX Enterprise server and the Asterisk PBX on the same LAN.
* G.711 is a voice codec and does not offer protection from jitter/packet loss/latency like T.38. If you are going this route, please make sure your network is reliable and stable before attempting to sends fax calls via G.711.
Furthermore, Fax over IP does not function properly over Voice over IP (VoIP) trunks.