The T.38 can be used as a FoIP protocol by itself, running over UDP. It is also adapted to be included as image / T.38 in a SDP attached to a SIP Invite - this way running as a "codec".?
The following products that are currently supporting the T.38 fax protocol:
Note: The PAP2 and the PAP2T do NOT support the T.38 fax protocol.
Setting-Up your Fax Machine?
Step 1: Log into the SPA's Web-Based setup page and click the Line tab you want to setup your fax machine.
Step 2: Scroll down to the bottom of the screen and look for Audio Configuration.
By default, the selection for fax is already enable and set to YES.? Make sure you select the proper codec need it for your Fax transmission which only G.711u or G.711a are the only option that can be used, when selecting the Fax Passthru Codec.
Make sure when connecting a fax machine to your SPA, that you select ReINVITE next to Fax Passthru Method. And make sure you have the T.38 option set to YES.