T.38 can be used in the following ways on a digital system:
- T.38 defines a method for a fax end-point (a fax machine or gateway) to transmit a fax over a network using the UDP protocol natively. This does not fit into Asterisk’s means of transmitting data as it describes a method of exchanging data that is unique to the T.38 protocol. Using this method would basically take the PABX out of the loop entirely (which may be acceptable)
It may be possible to use a 3rd party T.38 service directly with a T.38 enabled ATA to achieve bi-directional Fax over the Internet, but this is beyond the remit of the PABX configuration.
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Asterisk is able to terminate a T.38 call, and the IPCortex PABX will automatically use T.38 if offered on a SIP trunk call that terminates on a soft-fax number. At present there is no facility to scan and send documents outbound from the PABX, only to receive and email them.
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T.38 can also be represented in a manner compatible with existing SIP devices. At present, Asterisk is only capable of acting as a pass-through agent for T.38 streams, so cannot convert it to other formats - Pass-through allows two endpoints which understand T.38 over RTP to exchange faxes. This might include two ATA devices within the same company, but excludes fax machines connected to an ordinary telephone line, so is generally not particularly useful to our customers.
The PABX does not currently support this mode, but we are constantly monitoring progress as this software improves. It is expected that the “Next generation” PABX release will include a T.38 gateway facility to convert from T.38 to regular fax and vice-versa. This should be particularly helpful to enable sending and receiving faxes using a fax machine over ISDN or over a T.38 enabled SIP trunk.