Twilio Programmable Fax works using an API for sending and receiving faxes. Currently there is no interface between the Twilio Fax API and FreePBX. You can of course try to build something, but I have not seen anything built yet.
Or use the Twilio Elastic SIP trunking for Twilio numbers designated as Voice (not Fax) and accept faxes in FreePBX over a SIP Trunk.