Twilio supports faxing .PDF documents over the T.30 protocol via the Twilio REST API. For more information, please see our article Getting Started with Twilio Programmable Fax.
At this time, we have no plans to support or migrate to the T.38 fax protocol.
Notice: Twilio Programmable Fax will be disabled for all users on December 17, 2021. New and inactive accounts will lose access as of December 17, 2020. For full details, including migration options, see Fax Support on Twilio.