Before Exchange will receive an inbound fax, you must first update the a field in the Exchange UM server’s bin\globcfg.xml to True, and restart UM.
EnableInbandFaxDetection: Specifies whether inband fax detection is enabled
or not. The default setting is false and UM relies on the IP Gateways to
perform the FAX detection.
That will, instead of waiting for the IP PBX to send a T.38 request, or a RFC2833 fax tone, UM actually will try to listen to the raw fax tone in the audio stream.