I’ve read that asterisk 1.4 now supports T.38 protocol for faxes. Before I was using faxes with asterisk 1.2 and everything was working quite well. Could anyone explain how everything was working before. Another question is do I have to have T.38 supported by my ATA and my SIP provider or it’s enough that it is supported by asterisk.
Thanks in advance
faxing works like a modem, by scanning the page into data, and sending this data as audio tones over the phone line.
With ‘normal’ faxing, the modem’s audio data is sent as standard ulaw/alaw audio at 64kbit/sec. Asterisk treats it no differently than any other voice call, and the audio is reassembled on the other side. This way, Asterisk acts like a normal phone line.
This often works, and often doesn’t. Little things like jitter or packet loss that a human wouldn’t notice can interrupt or cause errors with the data session.
T.38 provides a solution to this problem. Instead of transmitting the raw audio, the ATA device has a ‘modem’ which decodes it into the underlying data stream. This data is sent over VoIP as data (NOT audio) and the other ATA turns it back into modem tones. This makes it more reliable because the modem’s carrier signal is generated by each ATA and only the data is transmitted- thus issues like latency or jitter can be easily sidestepped.
As I understand it Asterisk doesn’t so much ‘support’ T.38 as correctly pass it through. Asterisk cannot generate or decode T.38, so yes you will need gateways or ATAs that support T.38 on both sides.
I think this page can help you a lot, or may be you’ve read this already:
voip-info.org/wiki-Asterisk+fax
Thanks very much 
While Asterisk only has T.38 passthrough support (purely to to licensing issues), OpenPBX http://openpbx.org/, which is a fork of Asterisk, has full T.38 support.