This is because the fax is transmitted using the audio stream. It is not related to the signaling protocol (SIP/IAX etc.) but to the audio codec used.
Fax uses FSK modulation to transmit the data. If you compress this in a lossy way (GSM, MP3, whatever) then the integrity of the data is affected (more or less seriously depending on the codec used). Fax machines are generally quite picky, so compressing faxes is unlikely to work.
I'm wondering why on earth you want to push fax data over a VoIP link at all. Fax compression isn't very efficient. It would be much less bandwidth intensive to decode the fax and send it over as proper data rather than audio, compressed using gzip/gif/png/something else.
Alastair _______________________________________________ Asterisk-Users mailing list [EMAIL PROTECTED] http://lists.digium.com/mailman/listinfo/asterisk-users
