You can try to set the FXS port to "Common area phone" and see if you will receive faxes. Configure > Telephony > Ports and Trunks > FXS Ports > "Common area phone". FXS ports assigned to this role are configured on the Analog Extensions tab in the Users and Phones window ( Configure > Telephony > Users and Extensions > User and Phones).
Please try and tell if this is working for you. If it is not working please check what is your voice and your fax protocol over the SIP trunk.
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