What is T.38 and how does it work with Fax For Asterisk?
XT.38 is a specification for transmitting faxes across IP networks. Asterisk 1.4 provides the capability to allow one T.38 enabled endpoint to setup a call to another T.38 enabled endpoint for the purposes of fax transmission. Asterisk 1.4 is not, though, capable of terminating those T.38 calls into Asterisk; they can only be passed through. Asterisk 1.6 provides the capability to terminate a T.38 call into Asterisk. Fax For Asterisk may be used to establish a fax session using T.38 between Asterisk and a T.38-enabled endpoint or VoIP service provider. Asterisk and Fax For Asterisk do not support T.38 relay, meaning that a fax session from a T.38 enabled endpoint cannot be directed in real-time to a fax machine on the PSTN. In order to accomplish this, it is necessary to use Fax For Asterisk to first receive one leg of the fax into a TIFF file, and then upon its completion, establish a second call and transmit the TIFF file to the remote fax machine.
